Abstract:
An optical switching device includes a substrate having an optical waveguide thereon, and a phase transition pattern in or on a portion of the optical waveguide. The phase transition pattern includes a material that is configured to be switched between insulating and conductive states responsive to a stimulus. At least one conductive element on the substrate directly contacts the phase transition pattern to provide the stimulus to the phase transition pattern. Related fabrication methods are also discussed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2013-0092073 filed on Aug. 2, 2013, the disclosure of which is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Embodiments of the inventive concept relate to optical switching devices and methods of fabricating the same. 
         [0004]    2. Description of Related Art 
         [0005]    Optical switching devices are devices that receive/transmit data using light. An optical switching device may receive/transmit data by turning on/off light within a waveguide that transmits an optical signal. Various proposals and research on high-speed optical switching devices and methods of fabricating the same are progressing. 
       SUMMARY 
       [0006]    Embodiments of the inventive concept provide optical switching devices having a phase transition pattern. 
         [0007]    Embodiments of the inventive concept also provide methods of fabricating an optical switching device having a phase transition pattern. 
         [0008]    Embodiments of the inventive concept also provide semiconductor modules and electronic systems that include an optical switching device having a phase transition pattern. 
         [0009]    The technical objectives of the inventive concept are not limited to the disclosure herein; other objectives may become apparent to those of ordinary skill in the art based on the following descriptions. 
         [0010]    According to some aspects of the inventive concept, an optical switching device includes a substrate having an optical waveguide, and a phase transition pattern within or on a portion of the optical waveguide. The phase transition pattern includes a material that is configured to be switched between insulating and conductive states responsive to a stimulus. At least one conductive element on the substrate directly contacts the phase transition pattern. The conductive element is configured to provide the stimulus to the phase transition pattern. 
         [0011]    In some embodiments, the conductive element may be confined outside of the optical waveguide and extends at least partially along a sidewall thereof. For example, the conductive element may be first and second slabs adjacent respective sidewalls of the optical waveguide. 
         [0012]    In some embodiments, the first and second slabs may include a same material as the optical waveguide. For example, the optical waveguide may be formed of silicon, the first and second slabs may be formed of doped silicon. The first and second slabs may have respective thicknesses less than those of the optical waveguide and/or the phase transition pattern. 
         [0013]    In some embodiments, a conductive lower electrode may extend within the optical waveguide between the phase transition pattern and the substrate. A conductive upper electrode may extend on the phase transition pattern opposite the lower electrode. The lower electrode and the first and second slabs may be formed of silicon doped with an n-type or p-type impurity. 
         [0014]    In some embodiments, the phase transition pattern may be confined within the optical waveguide. In other embodiments, the phase transition pattern may be on one or more external surfaces of the optical waveguide. 
         [0015]    In accordance with an aspect of the inventive concept, an optical switching device includes a substrate including a trench, a lower cladding layer including an insulating material filled in the trench, an optical waveguide formed on the lower cladding layer, a phase transition pattern buried within a part of the optical waveguide to have the same thickness as a thickness of the optical waveguide, first and second slabs formed at both sides of the phase transition pattern and electrically connected to the phase transition pattern, and an upper cladding layer formed on the substrate and the lower cladding layer so as to cover the optical waveguide, the phase transition pattern, and the first and second slabs. 
         [0016]    In some embodiments, the phase transition pattern may include a vanadium oxide. 
         [0017]    In another embodiment, each of the first and second slabs may have a thickness that is less than a thickness of the phase transition pattern. 
         [0018]    In still another embodiment, the first and second slabs may be vertically disposed at the same level. 
         [0019]    In yet another embodiment, the first and second slabs may be vertically disposed at different levels. 
         [0020]    In yet another embodiment, the optical switching device may further include first and second slab via plugs directly contacting the first and second slabs by vertically penetrating the upper cladding layer, and first and second slab pads formed on the upper cladding layer so as to cover upper parts of the first and second slab via plugs. 
         [0021]    In accordance with another aspect of the inventive concept, an optical switching device includes: a substrate including a trench; a lower cladding layer including an insulating material filled in the trench; an optical waveguide formed on the lower cladding layer; a lower electrode region being in a part of the optical waveguide to have the same thickness as a thickness of the optical waveguide; first and second slabs formed at both sides of the phase transition pattern to a thickness that is less than a thickness of the lower electrode region; a phase transition pattern formed on the lower electrode region and parts of the first and second slabs; an upper electrode formed on a part of the phase transition pattern; and an upper cladding layer formed on the substrate and the lower cladding layer so as to cover the optical waveguide, the first and second slabs, the phase transition pattern, and the upper electrode. 
         [0022]    In some embodiments, the optical switching device may further include: a slab via plug directly contacting either of the first and second slabs by vertically penetrating the upper cladding layer; an upper electrode via plug directly contacting the upper electrode by vertically penetrating the upper cladding layer; a slab pad formed on the upper cladding layer so as to cover an upper part of the slab via plug; and an upper electrode pad formed on the upper cladding layer so as to cover an upper part of the upper electrode. 
         [0023]    In another embodiment, the upper electrode may include: a first upper electrode formed on parts of top and side surfaces of the phase transition pattern; and a second upper electrode spaced apart from the first upper electrode by a gap and formed on other parts of the top and side surfaces of the phase transition pattern. 
         [0024]    In still another embodiment, the optical switching device may further include an intermediate cladding layer formed on the substrate and the lower cladding layer, surrounding a part of both sides of the optical waveguide, and inserted between the lower cladding layer and the upper cladding layer. 
         [0025]    Detailed matters of other embodiments are included in the detailed description and the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The foregoing and other features and advantages of the inventive concepts will be apparent from the more particular description of preferred embodiments of the inventive concepts, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating principles of the inventive concepts. In the drawings: 
           [0027]      FIG. 1A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 1B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 1A ; 
           [0028]      FIG. 2A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 2B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 2A ; 
           [0029]      FIG. 3A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 3B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 3A ; 
           [0030]      FIG. 4A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 4B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 4A ; 
           [0031]      FIG. 5A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 5B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 5A ; 
           [0032]      FIGS. 6A through 6U  illustrate operations of a method of fabricating an optical switching device in accordance with some embodiments of the inventive concept and are longitudinal cross-sectional views taken along line I-I′ of  FIG. 1A ; 
           [0033]      FIGS. 7A through 7O  illustrate operations of a method of fabricating an optical switching device in accordance with some embodiments of the inventive concept and are longitudinal cross-sectional views taken along line I-I′ of  FIG. 3A ; 
           [0034]      FIGS. 8A through 8F  illustrate operations of a method of fabricating an optical switching device in accordance with some embodiments of the inventive concept and are longitudinal cross-sectional views taken along line I-I′ of  FIG. 4A ; 
           [0035]      FIG. 9  is a view of construction of a memory device including an optical switching device in accordance with some embodiments of the inventive concept; 
           [0036]      FIG. 10  is a view of construction of a semiconductor module including an optical switching device in accordance with some embodiments of the inventive concept; and 
           [0037]      FIG. 11  is a view of construction of an electronic device including an optical switching device in accordance with some embodiments of the inventive concept. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0038]    Various embodiments will now be described more fully with reference to the accompanying drawings in which some embodiments are shown. These inventive concepts may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough and complete and fully conveys the inventive concept to those skilled in the art. In the drawings, the sizes and relative sizes of layers and regions may be exaggerated for clarity. 
         [0039]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof. 
         [0040]    Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element&#39;s or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein may be interpreted accordingly. 
         [0041]    It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element. Thus, a first element discussed below could be termed a second element without departing from the scope of the present inventive concept. In addition, as used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It also will be understood that, as used herein, the term “comprising” or “comprises” is open-ended, and includes one or more stated elements, steps and/or functions without precluding one or more unstated elements, steps and/or functions. The term “and/or” includes any and all combinations of one or more of the associated listed items. 
         [0042]    It will also be understood that when an element is referred to as being “connected” to another element, it can be directly connected to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” to another element, there are no intervening elements present. It will also be understood that the sizes and relative orientations of the illustrated elements are not shown to scale, and in some instances they have been exaggerated for purposes of explanation. 
         [0043]    Embodiments are described herein with reference to cross-sectional and/or perspective illustrations that are schematic illustrations of idealized embodiments (and intermediate structures). As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the present inventive concept. 
         [0044]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this inventive concept belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
         [0045]      FIG. 1A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 1B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 1A . 
         [0046]    Referring to  FIGS. 1A and 1B , the optical switching device in accordance with the embodiment of the inventive concept may include a substrate  100 , a lower cladding layer  110 L, an optical waveguide  120 , a phase transition pattern  130 , first and second slabs  140  and  145 , and an upper cladding layer  110 U. The optical switching device in accordance with the embodiment of the inventive concept may further include first and second slab via plugs  150  and  155  and first and second slab pads  160  and  165 . 
         [0047]    The substrate  100  may be a bulk silicon wafer. The substrate  100  may include monocrystalline silicon. 
         [0048]    The lower cladding layer  110 L may be buried within the substrate  100 . The lower cladding layer  110 L may have a first horizontal width w1 in a first direction X and may extend in a second direction Y perpendicular to the first direction X. The lower cladding layer  110 L may include a silicon oxide. The lower cladding layer  110 L may have a lower refractive index than a refractive index of the optical waveguide  120  that will be described below. 
         [0049]    The optical waveguide  120  may be a transmission line that transmits an optical signal. The optical waveguide  120  may optically connect a coupler and a photoelectric converter. The optical waveguide  120  may be disposed on the lower cladding layer  110 L and may have a second horizontal width w2 that is less than the first horizontal width w1 in the first direction X. The optical waveguide  120  may have a first thickness t1 in a third direction Z perpendicular to planes formed in the first direction X and the second direction Y. The optical waveguide  120  may include monocrystalline silicon. The optical waveguide  120  may have a higher refractive index than the refractive index of the lower cladding layer  110 L and a refractive index of the upper cladding layer  110 U that will be described below. 
         [0050]    The phase transition pattern  130  may be buried or confined within the optical waveguide  120 . A horizontal width and a thickness of the phase transition pattern  130  are the same as or similar to those of the optical waveguide  120 . That is, the phase transition pattern  130  may have the second horizontal width w2 in the first direction X and may have the first thickness t1 in the third direction Z. The phase transition pattern  130  may have a length 1 in the second direction Y. The phase transition pattern  130  may be transformed into a metallic or conducting state from an insulating or non-conducting state and vice versa depending on the amount of a carrier injected into the phase transition pattern  130  due to a voltage or other stimulus applied to the phase transition pattern  130 . For example, when no voltage is applied to the phase transition pattern  130 , the phase transition pattern  130  may be in the insulating state, and when a voltage is applied to the phase transition pattern  130 , the phase transition pattern  130  may be in the metallic state. When the phase transition pattern  130  is in the insulating state, light that is being transmitted through the optical waveguide  120  may pass through the phase transition pattern  130  at an optical transmittance of about 90 to about 95%; that is, the phase transition pattern may be substantially transparent in the insulating state. On the other hand, when the phase transition pattern  130  is in the metallic state, light that is being transmitted through the optical waveguide  120  may be absorbed, scattered, and/or reflected by the injected carrier and may pass through the phase transition pattern  130  at an optical transmittance of about 5 to about 10%. Thus, the phase transition pattern  130  may operate as a switch that turns on/off (that is, allows or prevents passage of) light transmitted through the optical waveguide  120  due to a rapid change in optical transmittance depending on the insulating state and/or metallic state. For example, when the phase transition pattern  130  is in the insulating state, the phase transition pattern  130  may operate to turn on the light transmission, and when the phase transition pattern  130  is in the metallic state, the phase transition pattern  130  may operate to turn off the light transmission. The phase transition pattern  130  may include a vanadium oxide. 
         [0051]    The first and second slabs  140  and  145  may be formed at both sides of the phase transition pattern  130 , and in some embodiments, may be confined outside of the waveguide  120 . Each of the first and second slabs  140  and  145  may have a third horizontal width w3 that is less than the first horizontal width w1 and greater than the second horizontal width w2 in the first direction X (w1&gt;w3&gt;w2) and may have a second thickness t2 that is less than or equal to the first thickness t1 in the third direction Z (t1≧t2). Each of the first and second slabs  140  and  145  may have the same length as the length 1 of the phase transition pattern  130  in the second direction Y. Each of the first and second slabs  140  and  145  may be also formed in any position of both sides of the phase transition pattern  130  in the third direction Z. For example, the first and second slabs  140  and  145  may be formed in such a way that bottom surfaces of the first and second slabs  140  and  145  may be placed on the same horizontal plane as a bottom surface of the phase transition pattern  130 , as illustrated in  FIGS. 1A and 1B . In this case, good horizontal uniformity of injecting the carrier into the phase transition pattern  130  may be obtained. The first and second slabs  140  and  145  may be electrically connected to the phase transition pattern  130 . Each of the first and second slabs  140  and  145  may include doped silicon. Each of the first and second slabs  140  and  145  may include a P-type impurity and/or an N-type impurity. As such, the first and second slabs  140  and  145  may be conductive elements configured to apply a voltage or other stimulus to the phase transition pattern  130  to change the phase thereof. 
         [0052]    The upper cladding layer  110 U may be formed on the substrate  100 , the lower cladding layer  110 L, the optical waveguide  120 , the phase transition pattern  130 , and the first and second slabs  140  and  145  so as to have a thickness. The upper cladding layer  110 U may include a silicon oxide. The upper cladding layer  110 U and the lower cladding layer  110 L may include the same material so as to be materially connected to each other. The upper cladding layer  110 U may have a lower refractive index than the refractive index of the optical waveguide  120 . 
         [0053]    In the present embodiment, the optical waveguide  120 , the phase transition pattern  130 , and the first and second slabs  140  and  145  may be embedded or encapsulated by the lower cladding layer  110 L and the upper cladding layer  110 U. 
         [0054]    The first and second slab via plugs  150  and  155  may penetrate the upper cladding layer  110 U vertically and may directly contact the first and second slabs  140  and  145  so as to be electrically connected to the first and second slabs  140  and  145 . Each of the first and second slab via plugs  150  and  155  may include a metal or metal compound. Outer sides of the first and second slab via plugs  150  and  155  may be surrounded by the upper cladding layer  110 U formed of an insulating material. 
         [0055]    The first and second slab pads  160  and  165  may be formed on the upper cladding layer  110 U so as to contact and/or cover upper parts of the first and second slab via plugs  150  and  155 . The first and second slab pads  160  and  165  may include a metal or metal compound. 
         [0056]    In the optical switching device according to the present embodiment, when a voltage is applied to the optical switching device via the first and second slab pads  160  and  165 , the voltage may be applied to both left and right ends of the phase transition pattern  130  through the first and second slabs  140  and  145  via the first and second slab via plugs  150  and  155 . The amount of the carrier injected into the phase transition pattern  130  may be adjusted depending on a magnitude of the applied voltage. Thus, in the optical switching device according to the present embodiment, the phase transition pattern  130  is transformed into the metallic state from the insulating state or vice versa depending on the amount of the injected carrier, thereby turning on/off light transmitted through the optical waveguide  120 . 
         [0057]      FIG. 2A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 2B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 2A . The optical switching device illustrated in  FIGS. 2A and 2B  is similar to the optical switching device illustrated in  FIGS. 1A and 1B  except the position of a second slab  145 . Thus, detailed descriptions of the same elements will be omitted. 
         [0058]    Referring to  FIGS. 2A and 2B , the optical switching device in accordance with some embodiments of the inventive concept may include a substrate  100 , a lower cladding layer  110 L, an optical waveguide  120 , a phase transition pattern  130 , first and second slabs  14 Q and  145 , and an upper cladding layer  110 U. The optical switching device in accordance with some embodiments of the inventive concept may further include first and second slab via plugs  150  and  155  and first and second slab pads  160  and  165 . 
         [0059]    In the present embodiment, the first slab  140  may be formed in such a way that a bottom surface of the first slab  140  may be placed on the same horizontal plane as a bottom surface of the phase transition pattern  130 , and the second slab  145  may be formed in such a way that a top surface of the second slab  145  may be placed on the same horizontal plane as a top surface of the phase transition pattern  130 . In this case, good vertical uniformity of injecting the carrier into the phase transition pattern  130  may be obtained. 
         [0060]      FIG. 3A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 3B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 3A . 
         [0061]    Referring to  FIGS. 3A and 3B , the optical switching device in accordance with some embodiments of the inventive concept may include a substrate  100 , a lower cladding layer  110 L, an optical waveguide  120 , a lower electrode region  175 , first and second slabs  140  and  145 , a phase transition pattern  130 , an upper electrode  170 , and an upper cladding layer  110 U. The optical switching device in accordance with some embodiments of the inventive concept may further include an upper electrode via plug  180 , a lower electrode via plug  185 , an upper electrode pad  190 , and a lower electrode pad  195 . 
         [0062]    The substrate  100  may be a bulk silicon wafer. The substrate  100  may include monocrystalline silicon. 
         [0063]    The lower cladding layer  110 L may be buried within the substrate  100 . The lower cladding layer  110 L may have a first horizontal width w1 in a first direction X and may extend in a second direction Y perpendicular to the first direction X. The lower cladding layer  110 L may include a silicon oxide. The lower cladding layer  110 L may also have a lower refractive index than a refractive index of the optical waveguide  120  that will be described below. 
         [0064]    The optical waveguide  120  may be a transmission line that transmits an optical signal. The optical waveguide  120  may optically connect a coupler and a photoelectric converter. The optical waveguide  120  may be disposed on the lower cladding layer  110 L and may have a second horizontal width w2 that is less than the first horizontal width w1 in the first direction X. The optical waveguide  120  may have a first thickness t1 in a third direction Z perpendicular to a plane formed in the first direction X and the second direction Y. The optical waveguide  120  may include monocrystalline silicon. The optical waveguide  120  may have a higher refractive index than the refractive index of the lower cladding layer  110 L and a refractive index of the upper cladding layer  110 U that will be described below. 
         [0065]    The lower electrode region  175  may be formed to be buried or confined within a part of the optical waveguide  120 . A horizontal width and a thickness of the lower electrode region  175  are similar to those of the optical waveguide  120 . That is, the lower electrode region  175  may have the second horizontal width w2 in the first direction X and the first thickness t1 in the third direction Z. The lower electrode region  175  may have a length 1 in the second direction Y. The lower electrode region  175  may be electrically connected to the phase transition pattern  130  that will be described below. The lower electrode region  175  may include doped silicon. The lower electrode region  175  may include a P-type impurity and/or an N-type impurity. 
         [0066]    The first and second slabs  140  and  145  may be formed at both sides of the lower electrode region  175 . Each of the first and second slabs  140  and  145  may have a third horizontal width w3 that is less than the first horizontal width w1 and greater than the second horizontal width w2 in the first direction X (w1&gt;w3&gt;w2) and may have a second thickness t2 that is less than or equal to the first thickness t1 in the third direction Z (t1≧t2). Each of the first and second slabs  140  and  145  may have the same length as the length 1 of the lower electrode region  175  in the second direction Y. Each of the first and second slabs  140  and  145  may be also formed in any position at both sides of the lower electrode region  175  in the third direction Z. For example, the first and second slabs  140  and  145  may be formed in such a way that bottom surfaces of the first and second slabs  140  and  145  may be placed on the same horizontal plane as a bottom surface of the lower electrode region  175 , as illustrated in  FIGS. 3A and 3B . Each of the first and second slabs  140  and  145  may include the same material so as to be materially connected to the lower electrode region  175 . Each of the first and second slabs  140  and  145  may be electrically connected to the phase transition pattern  130  that will be described below. Each of the first and second slabs  140  and  145  may include doped silicon. Each of the first and second slabs  140  and  145  may include a P-type impurity and/or an N-type impurity, thereby defining conductive elements that are configured to apply a voltage or other stimulus to the phase transition pattern  130  to change the phase thereof. 
         [0067]    The phase transition pattern  130  may be formed partially or entirely on surfaces of the first and second slabs  140  and  145  and a surface of the lower electrode region  175 . In detail, the phase transition pattern  130  may be formed to surround parts of top surfaces of the first and second slabs  140  and  145  and parts of sides and a top surface of the lower electrode region  175 . The phase transition pattern  130  may have a horizontal width that is less than the first horizontal width w1 and greater than the second horizontal width w2 in the first direction X and may have a thickness in the third direction Z. The phase transition pattern  130  may have the same length as the length 1 of the lower electrode region  175  in the second direction Y. The phase transition pattern  130  may be transformed into a metallic state from an insulating state or vice versa depending on the amount of a carrier injected into the phase transition pattern  130  due to a voltage applied to the phase transition pattern  130 . For example, when no voltage is applied to the phase transition pattern  130 , the phase transition pattern  130  may be in the insulating state, and when a voltage is applied to the phase transition pattern  130 , the phase transition pattern  130  may be in the metallic state. When the phase transition pattern  130  is in the insulating state, light that is being transmitted through the optical waveguide  120  may pass through the phase transition pattern  130  at an optical transmittance of about 90 to about 95%. On the other hand, when the phase transition pattern  130  is in the metallic state, light that is being transmitted through the optical waveguide  120  may be absorbed, scattered, and/or reflected by the injected carrier and may pass through the phase transition pattern  130  at an optical transmittance of about 5 to about 10%. Thus, the phase transition pattern  130  may operate as a switch that turns on/off (that is, allows or prevents passage of) light transmitted through the optical waveguide  120  due to a rapid change in optical transmittance depending on the insulating state and/or metallic state. For example, when the phase transition pattern  130  is in the insulating state, the phase transition pattern  130  may operate to turn on the light transmission, and when the phase transition pattern  130  is in the metallic state, the phase transition pattern  130  may operate to turn off the light transmission. The phase transition pattern  130  may include a vanadium oxide. 
         [0068]    The upper electrode  170  may be formed partially or entirely on a part of a top surface of the phase transition pattern  130 . In this case, the upper electrode  170  may be formed to cover all of a top surface of the phase transition pattern  130  vertically aligned with the lower electrode region  175  or to cover only a part of the top surface of the phase transition pattern  130  vertically aligned with the first and second slabs  140  and  145 . The upper electrode  170  may have a horizontal width that is less than the horizontal width of the phase transition pattern  130  in the first direction X and may have a thickness in the third direction Z. The upper electrode  170  may have the same length as the length 1 of the phase transition pattern  130  in the second direction Y. The upper electrode  170  may be electrically connected to the phase transition pattern  130 . The upper electrode  170  may include a metal or metal compound. The upper electrode may include a transparent electrode, for example, an indium tin oxide (ITO). 
         [0069]    The upper cladding layer  110 U may be formed on the substrate  100 , the lower cladding layer  110 L, the optical waveguide  120 , the first and second slabs  140  and  145 , the phase transition pattern  130 , and the upper electrode  170  so as to have a thickness. The upper cladding layer  110 U may include a silicon oxide. The upper cladding layer  110 U and the lower cladding layer  110 L may include the same material so as to be materially connected to each other. The upper cladding layer  110 U may have a lower refractive index than the refractive index of the optical waveguide  120 . 
         [0070]    In the present embodiment, the optical waveguide  120 , the first and second slabs  140  and  145 , the lower electrode region  175 , the phase transition pattern  130 , and the upper electrode  170  may be embedded or encapsulated by the lower cladding layer  110 L and the upper cladding layer  110 U. 
         [0071]    The upper electrode via plug  180  may penetrate the upper cladding layer  110 U vertically and may directly contact the upper electrode  170  so as to be electrically connected to the upper electrode  170 . The upper electrode via plug  180  may include a metal or metal compound. Sides of the upper electrode via plug  180  may be surrounded by the upper cladding layer  110 U formed of an insulating material. 
         [0072]    The lower electrode via plug  185  may penetrate the upper cladding layer  110 U vertically and may directly contact the first and/or second slabs  140  and  145  so as to be electrically connected to either of the first and second slabs  140  and  145 . The lower electrode via plug  185  may include a metal or metal compound. Sides of the lower electrode via plug  185  may be surrounded by the upper cladding layer  110 U formed of an insulating material. 
         [0073]    The upper and lower electrode pads  190  and  195  may be formed on the upper cladding layer  110 U so as to contact and/or cover upper parts of the upper and lower electrode via plugs  180  and  185 . Each of the upper and lower electrode pads  190  and  195  may include a metal or metal compound. 
         [0074]    In the optical switching device according to the present embodiment, when a voltage is applied to the optical switching device through the upper and lower electrode pads  190  and  195 , the voltage may be applied to both top and bottom ends of the phase transition pattern  130  through the upper electrode  170  and the lower electrode region  175  via the upper and lower electrode via plugs  180  and  185 . The amount of the carrier injected into the phase transition pattern  130  may be adjusted depending on a magnitude of the applied voltage. Thus, in the optical switching device according to the present embodiment, the phase transition pattern  130  is transformed into the metallic state from the insulating state or vice versa depending on the amount of the injected carrier, thereby turning on/off light transmitted through the optical waveguide  120 . 
         [0075]      FIG. 4A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 4B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 4A . The optical switching device illustrated in  FIGS. 4A and 4B  is similar to the optical switching device illustrated in  FIGS. 3A and 3B  except first and second upper electrodes  171  and  172 , first and second upper electrode via plugs  181  and  182 , and first and second upper electrode pads  191  and  192 . Thus, detailed descriptions of the same elements will be omitted. 
         [0076]    Referring to  FIGS. 4A and 4B , the optical switching device in accordance with some embodiments of the inventive concept may include a substrate  100 , a lower cladding layer  110 L, an optical waveguide  120 , a lower electrode region  175 , first and second slabs  140  and  145 , a phase transition pattern  130 , first and second upper electrodes  171  and  172 , and an upper cladding layer  110 U. The optical switching device in accordance with some embodiments of the inventive concept may further include first and second upper electrode via plugs  181  and  182 , a lower electrode via plug  185 , first and second upper electrode pas  191  and  192 , and a lower electrode pad  195 . 
         [0077]    The first and second upper electrodes  171  and  172  may be formed on a part of a surface of the phase transition pattern  130  so as to be spaced apart from each other by a gap. For example, the first upper electrode  171  may be formed to surround parts of a top and side surfaces of the phase transition pattern  130 , e.g., an upper part of a left side and a part of a left top surface of the phase transition pattern  130 , and the second upper electrode  172  may be formed to surround other parts of the top and side surfaces of the phase transition pattern  130 , e.g., an upper part of a right side and a part of a right top surface of the phase transition pattern  130 . Each of the first and second upper electrodes  171  and  172  may have a horizontal width that is less than a half of the horizontal width of the phase transition pattern  130  in the first direction X and may have a thickness in the third direction Z. Each of the first and second upper electrodes  171  and  172  may have the same length as a length 1 of the phase transition pattern  130  in the second direction Y. The first and second upper electrodes  171  and  172  may be electrically connected to the phase transition pattern  130 . Each of the first and second upper electrodes  171  and  172  may include a metal or metal compound. Each of the first and second upper electrodes  171  and  172  may include a transparent electrode, for example, an ITO. 
         [0078]    The first and second upper electrode via plugs  181  and  182  may penetrate the upper cladding layer  110 U vertically and may directly contact the first and second upper electrodes  171  and  172  so as to be electrically connected to the first and second upper electrodes  171  and  172 . Each of the first and second upper electrode via plugs  181  and  182  may include a metal or metal compound. Sides of the first and second upper electrode via plugs  181  and  182  may be surrounded by the upper cladding layer  110 U formed of an insulating material. 
         [0079]    The first and second upper electrode pads  191  and  192  may be formed on the upper cladding layer  110 U so as to contact and/or cover top surfaces of the first and second upper electrode via plugs  181  and  182 . Each of the first and second upper electrode pads  191  and  192  may include a metal or metal compound. 
         [0080]    In the optical switching device according to the present embodiment, when a voltage is applied to the optical switching device through a node in which the first and second upper electrode pads  191  and  192  are electrically connected to each other and the lower electrode pad  195 , the voltage may be applied to both top and bottom ends of the phase transition pattern  130  through the first and second upper electrode via plugs  181  and  182  and the lower electrode via plug  185 . The amount of a carrier injected into the phase transition pattern  130  may be adjusted depending on a magnitude of the applied voltage. Thus, in the optical switching device according to the present embodiment, the phase transition pattern  130  is transformed into the metallic state from the insulating state or vice versa depending on the amount of the injected carrier, thereby turning on/off light transmitted through the optical waveguide  120 . 
         [0081]      FIG. 5A  is a partial perspective view schematically illustrating an optical switching device in accordance with some embodiments of the inventive concept, and  FIG. 5B  is a longitudinal cross-sectional view taken along line I-I′ illustrated in  FIG. 5A . The present embodiment is a modified example of the optical switching device illustrated in  FIGS. 4A and 4B . The optical switching device illustrated in  FIGS. 5A and 5B  is similar to the optical switching device of  FIGS. 4A and 4B  except that a lower electrode region  175  that serves as a lower electrode and first and second slabs  140  and  145  are omitted, except positions at which a phase transition pattern  130  and first and second upper electrodes  171  and  172  are to be formed due to this omission, and except insertion of an intermediate cladding layer  110 M between an upper cladding layer  110 U and a lower cladding layer  110 L. Thus, detailed descriptions of the same elements will be omitted. 
         [0082]    Referring to  FIGS. 5A and 5B , the optical switching device in accordance with some embodiments of the inventive concept may include a substrate  100 , a lower cladding layer  110 L, an optical waveguide  120 , a phase transition pattern  130 , an intermediate cladding layer  110 M, first and second upper electrodes  171  and  172 , and an upper cladding layer  110 U. The optical switching device in accordance with some embodiments of the inventive concept may further include first and second upper electrode via plugs  181  and  182  and first and second upper electrode pads  191  and  192 . 
         [0083]    The phase transition pattern  130  may be formed on a part of a surface of the optical waveguide  120 . In detail, the phase transition pattern  130  may be formed to surround a part of a top surface and a part of upper parts of both sides of the optical waveguide  120 . That is, the phase transition pattern  130  may have a horizontal width that is greater than or nearly equal to a horizontal width w2 of the optical waveguide  120  in the first direction X and may have a thickness in the third direction Z. The phase transition pattern  130  may have a length 1 in the second direction Y. 
         [0084]    The intermediate cladding layer  110 M may be formed on the substrate  100  and the lower cladding layer  110 L so as to have a thickness that is less than a first thickness t1 of the optical waveguide  120 . That is, the intermediate cladding layer  110 M may be formed to surround lower parts of both sides of the optical waveguide  120 . The intermediate cladding layer  110 M may include a silicon oxide. The intermediate cladding layer  110 M and the lower cladding layer  110 L may include the same material so as to be materially connected to each other. The intermediate cladding layer  110 M may have a lower refractive index than a refractive index of the optical waveguide  120 . 
         [0085]    The first and second upper electrodes  171  and  172  may be formed on parts of surfaces of the intermediate cladding layer  110 M and the phase transition pattern  130  so as to be spaced apart from each other by a gap. For example, the first upper electrode  171  may be formed on the intermediate cladding layer  110 M so as to surround parts of a top and side surfaces of the phase transition pattern  130 , e.g., an upper part of a left side and a part of a left top surface of the phase transition pattern  130 , and the second upper electrode  172  may be formed on the intermediate cladding layer  110 M so as to surround other parts of the top and side surfaces of the phase transition pattern  130 , e.g., an upper part of a right side and a part of a right top surface of the phase transition pattern  130 . Each of the first and second upper electrodes  171  and  172  may include a metal or metal compound. Each of the first and second upper electrodes  171  and  172  may include a transparent electrode, for example, an ITO. 
         [0086]    The upper cladding layer  110 U may be formed on the intermediate cladding layer  110 M, the first and second upper electrodes  171  and  172 , and the phase transition pattern  130  so as to have a thickness. The upper cladding layer  110 U may include a silicon oxide. The intermediate cladding layer  110 M and the upper cladding layer  110 U may include the same material so as to be materially connected to each other. The upper cladding layer  110 U may have a lower refractive index than the refractive index of the optical waveguide  120 . In  FIG. 5B , the intermediate cladding layer  110 M and the upper cladding layer  110 U are marked by chain thin lines so as to differentiate therebetween. 
         [0087]    In the present embodiment, the optical waveguide  120 , the phase transition pattern  130 , and the first and second upper electrodes  171  and  172  may be embedded or encapsulated by the lower cladding layer  110 L, the intermediate cladding layer  110 M, and the upper cladding layer  110 U. 
         [0088]    In the optical switching device according to the present embodiment, when a voltage is applied to the optical switching device through the first and second upper electrode pads  191  and  192 , the voltage may be applied to both left and right ends of the phase transition pattern  130  through the first and second upper electrode via plugs  181  and  182 . The amount of a carrier injected into the phase transition pattern  130  may be adjusted depending on a magnitude of the applied voltage. Thus, in the optical switching device according to the present embodiment, the phase transition pattern  130  is transformed into the metallic state from the insulating state or vice versa depending on the amount of the injected carrier, thereby turning on/off light transmitted through the optical waveguide  120 . 
         [0089]      FIGS. 6A through 6U  illustrate operations of a method of fabricating an optical switching device in accordance with some embodiments of the inventive concept and are longitudinal cross-sectional views taken along line I-I′ of  FIG. 1A . 
         [0090]    Referring to  FIG. 6A , the method of fabricating an optical switching device in accordance with the embodiment of the inventive concept may include forming pad mask layers  200  and a first mask pattern PR1 on a substrate  100 . The pad mask layers  200  may be formed partially or entirely on the substrate  100 . The substrate  100  may be a bulk silicon wafer. The substrate  100  may include monocrystalline silicon. The pad mask layers  200  may include a silicon nitride, a silicon oxide, or a combination thereof. For example, the pad mask layers  200  may include a relatively thin silicon oxide layer  210 , a relatively thicker silicon nitride layer  220 , and a relatively thicker silicon oxide layer  230 , which are formed on the substrate  100 . The first mask pattern PR1 may define a trench T to be formed to provide a space in which the optical switching device according to embodiments of the inventive concept is to be formed. The first mask pattern PR1 may include an organic material, for example, a photoresist. The first mask pattern PR1 may include an inorganic material having etching selectivity with respect to the pad mask layers  200 . 
         [0091]    Referring to  FIG. 6B , the method may include forming the trench T to be recessed into the substrate  100  by performing an etching process in which the first mask pattern PR1 is used as a patterning mask. A depth of the trench T may be about 1 micrometer (μm). The trench T may have a first horizontal width w1 in a first direction X and may extend in a second direction Y perpendicular to the first direction X. 
         [0092]    Referring to  FIG. 6C , the method may include forming a lower cladding material layer  10  by performing high density plasma chemical vapor deposition (HDP-CVD) and by filling an inside of the trench T with a lower cladding material. In this case, the lower cladding material layer  10  may be formed to fully fill the inside of the trench T and to sufficiently cover top surfaces of the pad mask layers  200 . The lower cladding material layer  10  may include an insulating material. The lower cladding material layer  10  may include a silicon oxide. 
         [0093]    Referring to  FIGS. 6D through 6F , the method may include exposing the surface of the substrate  100  by performing a planarization process. In this process, the lower cladding material that remains in the trench T may be transformed into a lower cladding layer  110 L. In detail, referring to  FIG. 6D , the method may include exposing the surface of the silicon nitride layer  220  of the pad mask layers  200  by performing a chemical mechanical polishing (CMP) process. Subsequently, referring to  FIG. 6E , the method may include removing the silicon nitride layer  220  by performing a wet etching process. Thereafter, referring to  FIG. 6F , the method may include removing the silicon oxide layer  210  and exposing the surface of the substrate  100  by performing a dry etching or wet etching process. In the processes of  FIGS. 6D through 6F , a top surface of the lower cladding layer  110 L may be placed at a level that is the same as or similar to a level of a top surface of the substrate  100 . The lower cladding layer  110 L may include a silicon oxide. 
         [0094]    Referring to  FIG. 6G , the method may include partially or entirely forming an amorphous material layer  12   a  on surfaces of the substrate  100  and the lower cladding layer  110 L. The amorphous material layer  12   a  may be formed on the substrate  100  and the lower cladding layer  110 L to a sufficient thickness by performing a deposition process. That is, the thickness of the amorphous material layer  12   a  may be a first thickness t1 in a third direction Z perpendicular to a plane formed in the first direction X and the second direction Y. The amorphous material layer  12   a  may include amorphous silicon or polycrystalline silicon. 
         [0095]    Referring to  FIG. 6H , the method may include monocrystallizing the amorphous material layer  12   a  into a monocrystalline material layer  12   c . The monocrystallization process may be performed using laser, thermal treatment, a rapid thermal process (RTP), and/or an annealing process using a furnace. The monocrystallization process using laser may be faster than other processes and may have a wide range of monocrystallization. According to embodiments of the inventive concept, surfaces of the substrate  100  may be used as a monocrystallization seed. 
         [0096]    Referring to  FIG. 6I , the method may include forming a second mask pattern PR2 on the monocrystalline material layer  12   c  and forming a phase transition pattern cavity or space St to be recessed into the monocrystalline material layer  12   c  by performing an etching process in which the second mask pattern PR2 is used as a patterning mask, so that a part of a surface of the lower cladding layer  110 L may be exposed. The phase transition pattern space St may have a second horizontal width w2 that is less than the first horizontal width w1 in the first direction X and may have the first thickness t1 in the third direction Z. The phase transition pattern space St may have a length 1 in the second direction Y by further referring to  FIG. 1A . The second mask pattern PR2 may include an organic material, for example, a photoresist. 
         [0097]    Referring to  FIG. 6J , the method may include forming a phase transition material layer  13  by removing the second mask pattern PR2, performing a deposition process to fill an inside of the phase transition pattern space St with a phase transition material. In this case, the phase transition material layer  13  may be formed to fully fill the inside of the phase transition pattern space St and to sufficiently cover the top surface of the monocrystalline material layer  12   c . The phase transition material layer  13  may include a vanadium oxide. 
         [0098]    Referring to  FIG. 6K , the method may include exposing the surface of the monocrystalline material layer  12   c  by performing the planarization process, such as CMP. In this process, the phase transition material that remains in the phase transition pattern space St may be transformed into or otherwise defines a phase transition pattern  130 . Thus, the phase transition pattern  130  may have the second horizontal width w2 in the first direction X, may have the first thickness t1 in the third direction Z, and may have a length 1 (see  FIG. 1A ) in the second direction Y. The phase transition pattern  13 Q may include a vanadium oxide. 
         [0099]    Referring to  FIG. 6L , the method may include forming a third mask pattern PR3 on a part of the monocrystalline material layer  12   c  and the phase transition pattern  130 . The third mask pattern PR3 may define an optical waveguide  120  and first and second preliminary slabs  140   p  and  145   p  (see  FIG. 6M ). The third mask pattern PR3 may include an organic material, for example, a photoresist. 
         [0100]    Referring to  FIG. 6M , the method may include forming the optical waveguide  120  and the first and second preliminary slabs  140   p  and  145   p  by removing a part of the monocrystalline material layer  12   c  so that the surfaces of the lower cladding layer  110 L and the substrate  100  may be exposed using an etching process in which the third mask pattern PR3 is used as a patterning mask. The optical waveguide  120  may have the second horizontal width w2 in the first direction X and may have the first thickness t1 in the third direction Z. The optical waveguide  120  may extend in the second direction Y by further referring to  FIG. 1A . The first and second preliminary slabs  140   p  and  145   p  may be formed at both sides of the phase transition pattern  130  so as to have a horizontal width in the first direction X. For example, each of the first and second preliminary slabs  140   p  and  145   p  may have a third horizontal width w3 that is less than the first horizontal width w1 and greater than the second horizontal width w2 in the first direction X. Each of the first and second preliminary slabs  140   p  and  145   p  may have the first thickness t1 in the third direction Z and may have the same length as the length 1 (see  FIG. 1A ) of the phase transition pattern  130  in the second direction Y. The optical waveguide  120  and the first and second preliminary slabs  140   p  and  145   p  may include the same material so as to be materially connected to each other. 
         [0101]    Referring to  FIG. 6N , the method may include forming a fourth mask pattern PR4 having a first opening O1 corresponding to the first and second preliminary slabs  140   p  and  145   p  on the substrate  100 , the lower cladding layer  110 L, the optical waveguide  120 , and the phase transition pattern  130 . The fourth mask pattern PR4 may include an organic material, for example, a photoresist. 
         [0102]    Referring to  FIG. 6O , the method may include forming first and second slabs  140  and  145  by recessing upper parts of the first and second preliminary slabs  140   p  and  145   p  using an etching process in which the fourth mask pattern PR4 is used as a patterning mask. The first and second slabs  140  and  145  may have the third horizontal width w3 in the first direction X and may have a second thickness t2 that is less than the first thickness t1 in the third direction Z. Each of the first and second slabs  140  and  145  may have the same length as the length 1 of the phase transition pattern  130  in the second direction Y by further referring to  FIG. 1A . In this process, a part of sides of the phase transition pattern  130  may be exposed. 
         [0103]    Referring to  FIG. 6P , the method may include forming a fifth mask pattern PR5 on the substrate  100 , the lower cladding layer  110 L, the optical waveguide  120 , and the phase transition pattern  130 , wherein the fifth mask pattern PR5 has a second opening O2 with a horizontal width in the first direction X that is less than that of the first opening O1, the second opening O2 corresponds to the first and second slabs  140  and  145 , and the fifth mask pattern PR5 surrounds the exposed upper part and part of sides of the phase transition pattern  130 . The fifth mask pattern PR5 may include an organic material, for example, a photoresist. 
         [0104]    Referring to  FIG. 6Q , the method may include doping impurities into the exposed first and second slabs  140  and  145  using the fifth mask pattern PR5. Each of the first and second slabs  140  and  145  may include doped silicon. Forming the first and second slabs  140  and  145  may include an ion implantation process. The first and second slabs  140  and  145  may be electrically connected to the phase transition pattern  130 . Each of the first and second slabs  140  and  145  may include a P-type impurity and/or an N-type impurity, defining conductive elements that are configured to apply a voltage or other stimulus to the phase transition pattern  130  to change the phase thereof. 
         [0105]    Referring to  FIG. 6R , the method may include forming an upper cladding layer  110 U on the substrate  100 , the lower cladding layer  110 L, the optical waveguide  120 , the phase transition pattern  130 , and the first and second slabs  140  and  145  to a sufficient thickness by performing a deposition process. The upper cladding layer  110 U may include the same material so as to be materially connected to the lower cladding layer  110 L. The upper cladding layer  110 U may include a silicon oxide. 
         [0106]    Referring to  FIG. 6S , the method may include forming a sixth mask pattern PR6 for defining via plug regions corresponding to the first and second slabs  140  and  145  on the upper cladding layer  110 U. The sixth mask pattern PR6 may include an organic material, for example, a photoresist. 
         [0107]    Referring to  FIG. 6T , the method may include forming first and second slab via holes H1 through which surfaces of the first and second slabs  140  and  145  are exposed, by penetrating the upper cladding layer  110 U using an etching process in which the sixth mask pattern PR6 is used as a patterning mask. 
         [0108]    Referring to  FIG. 6U , the method may include forming first and second slab via plugs  150  and  155  to directly contact the first and second slabs  140  and  145  by filling each of insides of the first and second slab via holes H1 with a conductive material. Each of the first and second slab via plugs  150  and  155  may include a metal or metal compound. Outer sides of the first and second slab via plugs  150  and  155  may be surrounded by the upper cladding layer  110 U. 
         [0109]    Subsequently, by further referring to  FIG. 1B , the method may further include forming first and second slab pads  160  and  165  on the upper cladding layer  110 U so as to be electrically connected to the first and second slab via plugs  150  and  155 . Each of the first and second slab pads  160  and  165  may include a metal or metal compound. 
         [0110]      FIGS. 7A through 7O  illustrate operations of a method of fabricating an optical switching device in accordance with some embodiments of the inventive concept and are longitudinal cross-sectional views taken along line I-I′ of  FIG. 3A . 
         [0111]    Referring to  FIGS. 6A through 6H  and  FIG. 7A , the method of fabricating an optical switching device in accordance with some embodiments of the inventive concept may include forming a seventh mask pattern PR7 for defining an optical waveguide  120  and first and second preliminary slabs  140   p  and  145   p  on the monocrystalline material layer  12   c . The seventh mask pattern PR7 may include an organic material, for example, a photoresist. 
         [0112]    Referring to  FIG. 7B , the method may include forming the optical waveguide  120  and the first and second preliminary slabs  140   p  and  145   p  by performing an etching process in which the seventh mask pattern PR7 is used as a patterning mask. The optical waveguide  120  may have a second horizontal width w2 that is less than the first horizontal width w1 in the first direction X and may have the first thickness t1 in the third direction Z. The optical waveguide  120  may extend in the second direction Y by further referring to  FIG. 3A . Each of the first and second preliminary slabs  140   p  and  145   p  may be formed to have a horizontal width in the first direction X. For example, each of the first and second preliminary slabs  140   p  and  145   p  may have a third horizontal width w3 that is less than the first horizontal width w1 and greater than the second horizontal width w2 in the first direction X. Each of the first and second preliminary slabs  140   p  and  145   p  may have a length 1 in the second direction Y by further referring to  FIG. 3A . The optical waveguide  120  and the first and second preliminary slabs  140   p  and  145   p  may include the same material so as to be materially connected to each other. 
         [0113]    Referring to  FIG. 7C , the method may include forming an eighth mask pattern PR8 having a first opening O1 corresponding to the first and second preliminary slabs  140   p  and  145   p  on the substrate  100 , the lower cladding layer  110 L, and the optical waveguide  120 . The eighth mask pattern PR8 may include an organic material, for example, a photoresist. 
         [0114]    Referring to  FIG. 7D , the method may include forming first and second slabs  140  and  145  by recessing upper parts of the first and second preliminary slabs  140   p  and  145   p  using an etching process in which the eighth mask pattern PR8 is used as a patterning mask. Each of the first and second slabs  140  and  145  may have the third horizontal width w3 that is less than the first horizontal width w1 and greater than the second horizontal width w2 in the first direction X, may have a second thickness t2 that is less than the first thickness t1 in the third direction Z, and may have the length 1 (see  FIG. 3A ) in the second direction Y. 
         [0115]    Referring to  FIG. 7E , the method may include forming a ninth mask pattern PR9 having a second opening O2 corresponding to the first and second slabs  140  and  145  and a part of the optical waveguide  120  (hereinafter, referred to as a lower electrode region  175 ) that corresponds to the length 1 (see  FIG. 3A ) in the second direction Y of the first and second slabs  140  and  145  on the substrate  100 , the lower cladding layer  110 L, and the optical waveguide  120 . The ninth mask pattern PR9 may include an organic material, for example, a photoresist. 
         [0116]    Referring to  FIG. 7F , the method may include doping impurities into the exposed first and second slabs  140  and  145  and the lower electrode region  175  using the ninth mask pattern PR9. The first and second slabs  140  and  145  and the lower electrode region  175  may include the same material so as to be materially connected to each other. The lower electrode region  175  may have the second horizontal width w2 in the first direction X, the first thickness t1 in the third direction Z, and the same length as the length 1 (see  FIG. 3A ) of each of the first and second slabs  140  and  145  in the second direction Y. Each of the first and second slabs  140  and  145  and the lower electrode region  175  may include doped silicon. Each of the first and second slabs  140  and  145  and the lower electrode region  175  may include a P-type impurity and/or an N-type impurity to define conductive elements that are configured to apply a voltage or other stimulus to the phase transition pattern  130  to change the phase thereof. Forming the first and second slabs  140  and  145  and the lower electrode region  175  may include an ion implantation process. 
         [0117]    Referring to  FIG. 7G , the method may include partially or entirely forming a phase transition material layer  13  and an upper electrode layer  17  on surfaces of the substrate  100 , the lower cladding layer  110 L, the first and second slabs  140  and  145 , and the lower electrode region  175 . 
         [0118]    Referring to  FIG. 7H , the method may include forming a tenth mask pattern PR10 that is disposed perpendicular to the lower electrode region  175  and parts of the first and second slabs  140  and  145  on the upper electrode layer  17 . The tenth mask pattern PR10 may include an organic material, for example, a photoresist. 
         [0119]    Referring to  FIG. 7I , the method may include forming an upper electrode  170  by etching the upper electrode layer  17  by performing an etching process in which the tenth mask pattern PR10 is used as a patterning mask. The upper electrode  170  may include a metal or metal compound. The upper electrode  170  may include a transparent electrode, for example, an ITO. 
         [0120]    Referring to  FIG. 7J , the method may include forming an eleventh mask pattern PR11 for covering the upper electrode  170  on the phase transition material layer  13 . The eleventh mask pattern PR11 may include an organic material, for example, a photoresist. 
         [0121]    Referring to  FIG. 7K , the method may include forming a phase transition pattern  130  and exposing the first and second slabs  140  and  145  by etching the phase transition material layer  13  by performing an etching process in which the eleventh mask pattern PR11 is used as a patterning mask. The phase transition pattern  130  may include a vanadium oxide. Although the phase transition pattern  130  and the upper electrode  170  are formed in the form of stairs on the first and second slabs  140  and  145 , aspects of the inventive concept are not limited thereto, and sides of the phase transition pattern  130  and the upper electrode  170  may have the same horizontal width in the first direction X. 
         [0122]    Referring to  FIG. 7L , the method may include forming an upper cladding layer  110 U for covering the first and second slabs  140  and  145 , the phase transition pattern  130 , and the upper electrode  170  on the substrate  100  and the lower cladding layer  110 L by performing a deposition process. The upper cladding layer  110 U may include the same material so as to be materially connected to the lower cladding layer  110 L. The upper cladding layer  110 U may include a silicon oxide. 
         [0123]    Referring to  FIG. 7M , the method may include a twelfth mask pattern PR12 for defining via plug regions corresponding to either of the first and second slabs  140  and  145  and the upper electrode  170  on the upper cladding layer  110 U. The twelfth mask pattern PR12 may include an organic material, for example, a photoresist. 
         [0124]    Referring to  FIG. 7N , the method may include forming a slab via hole H1 to directly contact either of the first and second slabs  140  and  145  and an upper electrode via hole H2 to directly contact the upper electrode  170  by penetrating the upper cladding layer  110 U using an etching process in which the twelfth mask pattern PR12 is used as a patterning mask. 
         [0125]    Referring to  FIG. 7O , the method may include forming a slab via plug  185  and an upper electrode via plug  180  by filling an inside of the slab via hole H1 and an inside of the upper electrode via hole H2 with a conductive material. Each of the slab via plug  185  and the upper electrode via plug  180  may include a metal or metal compound. Sides of the slab via plug  185  and the upper electrode via plug  180  may be surrounded by the upper cladding layer  110 U. 
         [0126]    Subsequently, further referring to  FIG. 3B , the method may further include forming a slab pad  195  and an upper electrode pad  190  to be respectively electrically connected to the slab via plug  185  and the upper electrode via plug  180  on the upper cladding layer  110 U. Each of the slab pad  195  and the upper electrode pad  190  may include a metal or metal compound. 
         [0127]      FIGS. 8A through 8F  illustrate operations of a method of fabricating an optical switching device in accordance with some embodiments of the inventive concept and are longitudinal cross-sectional views taken along line I-I′ of  FIG. 4A . 
         [0128]    Referring to  FIGS. 6A through 6H ,  FIGS. 7A through 7K , and  FIG. 8A , the method of fabricating an optical switching device in accordance with some embodiments of the inventive concept may include forming a thirteenth mask pattern PR13 having a third opening O3 through which a part of a top surface of the upper electrode  170  is exposed, on the substrate  100 , the lower cladding layer  110 L, the first and second slabs  140  and  145 , the phase transition pattern  130 , and the upper electrode  170 . The thirteenth mask pattern PR13 may include an organic material, for example, a photoresist. 
         [0129]    Referring to  FIG. 8B , the method may include exposing a part of a top surface of the phase transition pattern  130  by performing an etching process in which the thirteenth mask pattern PR13 is used as a patterning mask. In this process, the upper electrode  170  may be formed to be separated into first and second upper electrodes  171  and  172  that are spaced apart from each other by a gap. Each of the first and second upper electrodes  171  and  172  may include a metal or metal compound. Each of the first and second upper electrodes  171  and  172  may include a transparent electrode, for example, an ITO. 
         [0130]    Referring to  FIG. 8C , the method may include forming an upper cladding layer  110 U on the substrate  100  and the lower cladding layer  110 L by performing a deposition process so as to cover the first and second slabs  140  and  145 , the phase transition pattern  130 , and the first and second upper electrodes  171  and  172 . The upper cladding layer  110 U may include the same material so as to be materially connected to the lower cladding layer  110 L. The upper cladding layer  110 U may include a silicon oxide. 
         [0131]    Referring to  FIG. 8D , the method may include forming a fourteenth mask pattern PR14 for defining via plug regions corresponding to either of the first and second slabs  140  and  145  and the first and second upper electrodes  171  and  172  on the upper cladding layer  110 U. The fourteenth mask pattern PR14 may include an organic material, for example, a photoresist. 
         [0132]    Referring to  FIG. 8E , the method may include forming a slab via hole H1 to directly contact either of the first and second slabs  140  and  145  and upper electrode via holes H2 to directly contact the first and second upper electrodes  171  and  172  by penetrating the upper cladding layer  110 U using an etching process in which the fourteenth mask pattern PR14 is used as a patterning mask. 
         [0133]    Referring to  FIG. 8F , the method may include forming a slab via plug  185  and first and second upper electrode via plugs  181  and  182  by filling an inside of the slab via hole H1 and insides of the upper electrode via holes H2 with a conductive material. Each of the slab via plug  185  and the first and second upper electrode via plugs  181  and  182  may include a metal or metal compound. Sides of the slab via plug  185  and the first and second upper electrode via plugs  181  and  182  may be surrounded by the upper cladding layer  110 U. 
         [0134]    Subsequently, further referring to  FIG. 4B , the method may include forming a slab pad  195  and first and second upper electrode pads  191  and  192  to be electrically connected to the slab via plug  185  and the first and second upper electrode via plugs  181  and  182  on the upper cladding layer  110 U. Each of the slab pad  195  and the first and second upper electrode pads  191  and  192  may include a metal or metal compound. 
         [0135]    In accordance with the embodiment of the inventive concept, since light transmitted through the optical waveguide  120  may be turned on/off by the phase transition pattern  130 , which is formed within or on the optical waveguide  120  and a phase of which is transformed into a metallic state from an insulating state or vice versa depending on the amount of a carrier injected into the phase transition pattern  130 , high speed/low power consumption/miniaturization of the optical switching device may be achieved, and a degree of integration may be greatly improved. 
         [0136]      FIG. 9  is a view of construction of a memory device including an optical switching device in accordance with some embodiments of the inventive concept. 
         [0137]    Referring to  FIG. 9 , a memory device  1000  including the optical switching device in accordance with some embodiments of the inventive concept may include a memory substrate  1100 , a plurality of memory cells  1200 , and an input/output module  1300 . The memory substrate  1100  may be a bulk silicon wafer. 
         [0138]    The plurality of memory cells  1200  may be dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, phase change memory, magnetic random access memory (MRAM), and/or resistive random access memory (RRAM). 
         [0139]    The input/output module  1300  may include the optical switching device in accordance with some embodiments of the inventive concept. For example, the input/output module  1300  may include a waveguide buried in the memory substrate  1100 , an optical switching device, and/or an optical input/output device including a coupler and a photoelectric converter. Thus, the input/output module  1300  may have excellent reliability compared to the related art. 
         [0140]    The memory device  1000  may optically receive/transmit data from/to an adjacent electronic device or another memory device  1000  using the input/output module  1300  including the optical switching device in accordance with some embodiments of the inventive concept. Thus, the memory device  1000  may receive/transmit data stably and quickly. 
         [0141]      FIG. 10  is a view of construction of a semiconductor module including an optical switching device in accordance with some embodiments of the inventive concept. 
         [0142]    Referring to  FIG. 10 , a semiconductor module  2000  including the optical switching device in accordance with some embodiments of the inventive concept may include a module substrate  2100 , a plurality of semiconductor packages  2200 , a control chip package  2300 , and a plurality of input/output modules  2400 . The plurality of semiconductor packages  2200  and the control chip package  2300  may be electrically connected to the plurality of input/output modules  2400 . 
         [0143]    The plurality of semiconductor packages  2200  may include volatile memory chips, non-volatile memory chips, or a combination thereof. The volatile memory chips may include DRAM and/or SRAM. The non-volatile memory chips may include flash memory, phase change memory, MRAM, and/or RRAM. The semiconductor module  2000  may not include the control chip package  2300 . Thus, the semiconductor module  2000  including the optical switching device in accordance with some embodiments of the inventive concept may be a memory module. For example, the semiconductor module  2000  may be a memory card. 
         [0144]    The plurality of input/output modules  2400  may include the optical switching device in accordance with some embodiments of the inventive concept. For example, the plurality of input/output modules  2400  may include a waveguide buried in the module substrate  2100 , an optical switching device, and/or an optical input/output device including a coupler and a photoelectric converter. Thus, the plurality of input/output modules  2400  may have excellent reliability compared to the related art. 
         [0145]    The semiconductor module  2000  may optically receive/transmit data from/to an external electronic device using the plurality of input/output modules  2400  including the optical switching device in accordance with some embodiments of the inventive concept. Thus, the semiconductor module  2000  may receive/transmit data stably and quickly. 
         [0146]    Each of the plurality of semiconductor packages  2200  may include a memory device. That is, the plurality of semiconductor packages  2200  may include the optical switching device in accordance with some embodiments of the inventive concept. Thus, the plurality of semiconductor packages  2200  may be optically connected to each other. That is, the semiconductor module  2000  may cause data to be stably and quickly received/transmitted between the plurality of semiconductor packages  2200 . 
         [0147]      FIG. 11  is a view of construction of an electronic device including an optical switching device in accordance with some embodiments of the inventive concept. 
         [0148]    Referring to  FIG. 11 , an electronic device  4000  including the optical switching device in accordance with some embodiments of the inventive concept may include an interface  4100 , a controller  4200 , a memory  4300 , and an external input/output device  4400 . The interface  4100  may be electrically connected to the controller  4200 , the memory  4300 , and the external input/output device  4400  via a bus  4500 . 
         [0149]    The electronic device  4000  may be a personal digital assistant (PDA), a portable computer, a web tablet, a wireless phone, a mobile phone, and/or a digital music player. 
         [0150]    The interface  4100  may communicate data with an external system. That is, the interface  4100  may transmit/receive data to/from a communication network. The interface  4100  may include an optical input/output device in accordance with some embodiments of the inventive concept. The interface  4100  may communicate data with the external system using an optical signal. Thus, the interface  4100  may have excellent reliability compared to the related art. 
         [0151]    The controller  4200  may include a microprocessor, a digital processor, a microcontroller, or other similar processor devices. The external input/output device  4400  may be a keypad, a keyboard, or a display. 
         [0152]    The memory  4300  may be used to store a command executed by the controller  4200 . The memory  4300  may be optically connected to the interface  4100 . In this case, the memory  4300  may be a memory device including the optical switching device in accordance with some embodiments of the inventive concept. For example, the memory  4300  may include a waveguide buried in a memory substrate (not shown), an optical switching device, and/or an optical input/output device in which a coupler and a photoelectric converter are buried. Thus, data communication between the memory  4300  and the interface  4100  may show excellent reliability compared to the related art. 
         [0153]    The electronic device  4000  may optically receive/transmit data from/to the external system using the interface  4100  including the optical switching device in accordance with some embodiments of the inventive concept. Thus, the electronic device  4000  may receive/transmit data stably and quickly. 
         [0154]    As described above, in an optical switching device and a method of fabricating the same according to various embodiments of the inventive concept, a phase transition pattern is formed within or on an optical waveguide so as to turn on/off (i.e., allow or prevent passage of) light depending on the amount of a carrier injected therein so that high-speed/low power consumption/miniaturization of the optical switching device may be achieved and a degree of integration may be improved. 
         [0155]    The foregoing is illustrative of embodiments and is not to be construed as limiting thereof. Although a few embodiments have been described, those skilled in the art will readily appreciate that many modifications are possible in embodiments without materially departing from the novel teachings and advantages. Accordingly, all such modifications are intended to be included within the scope of this inventive concept as defined in the claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function, and not only structural equivalents but also equivalent structures. Therefore, it is to be understood that the foregoing is illustrative of various embodiments and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims.