Abstract:
Provided is an optical modulator including an optical waveguide and an optical modulation part integrated on the optical waveguide that is clad in oxide silicon and has silicon as core by using a bulk silicon wafer in place of an silicon-on-insulator (SOI) used for a typical optical waveguide and optical modulator and using complementary metal oxide semiconductor (CMOS) and thermal oxide film formation processes, and a fabrication method thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 of Korean Patent Application No. 10-2015-0006866, filed on Jan. 14, 2015, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure herein relates to an electro-absorption optical modulation device, and more particularly to, a silicon semiconductor electro-absorption optical modulation device and a method of fabricating the same. 
     In order to deal with a continuous increase in bandwidth according to a recent sharp increase in data communication, the miniaturization and stabilization of core optical parts are needed. 
     As a technology that may monolithically integrate an electronic circuit and an optical device by the application of a complementary metal oxide semiconductor (CMOS) process technology, a silicon photonics technology is significantly receiving attention. Thus, silicon or silicon on insulator (SOI) wafer based optical devices are being widely researched and developed and a technology that monolithically fabricates an active optical device and an electronic integrated circuit is being developed. 
     The silicon photonics technology shows a high use value in optical interconnection and typical optical communication. However, a generally used SOI wafer has constraints in the aspect of cost and technology. For example, it is expensive in comparison to a silicon wafer, and even when a CMOS IC is fabricated, it is difficult to apply a library for the silicon wafer. 
     The silicon photonics technology fabricating an SOI region locally on the silicon wafer is also receiving attention but it is true that integration is attempted by using an expensive SOI substrate for optical devices requiring an optical waveguide such as an optical modulator. 
     SUMMARY 
     The present disclosure provides an electro-absorption optical modulation device integrated on a bulk silicon wafer. 
     Tasks to be performed by the inventive concept are not limited to the above-mentioned tasks and other tasks not mentioned may be clearly understood by a person skilled in the art from the following descriptions. 
     An embodiment of the inventive concept provides an electro-absorption optical modulation device includes a semiconductor substrate, a first clad layer in a trench region formed in the semiconductor substrate, an optical waveguide core layer disposed spaced apart from sides of the trench region on the first clad layer and extended in a first direction, an optical modulation part disposed on the optical waveguide core layer and having an island shape, and a second clad layer covering the optical modulation part and the optical waveguide core layer. 
     In an embodiment, the semiconductor substrate may include a bulk silicon wafer. 
     In an embodiment, the optical modulation part may include a first contact layer, a depletion layer on the first contact layer, and a second contact layer on the depletion layer. The first contact layer may be extended to an outside of the depletion layer. The first contact layer may include a first semiconductor pattern, and the second contact layer may include a second semiconductor pattern. The depletion layer may include silicon (Si), germanium (Ge) or silicon germanium (SiGe). 
     In an embodiment, the optical waveguide core layer may be extended in a first direction. The first clad layer and the second clad layer may cover the optical modulation part and the optical waveguide core layer. The optical modulation part may have an island shape. A width of a second direction intersecting the first direction of the optical modulation part may be smaller than that of the second direction of the first clad layer. 
     In an embodiment, the first clad layer and the second clad layer may include silicon oxide or silicon nitride. 
     In an embodiment, the optical waveguide core layer may include single crystal silicon. 
     In an embodiments of the inventive concept, a method of fabricating an electro-absorption optical modulation device includes forming a first clad layer and an optical waveguide core layer on a surface of a semiconductor substrate, forming an optical modulation part on the optical waveguide core layer, and forming a second clad layer on the optical modulation part and the optical waveguide core layer, wherein the forming of the first clad layer and the optical waveguide core layer includes: filing a pair of first trenches with oxide, the first trenches being formed in a surface of the semiconductor substrate and extended in a first direction, and forming a second trench under the first trenches, the second trench being filled with oxide and connecting the first trenches. 
     In an embodiment, the forming of the first clad layer and the optical waveguide core layer may include forming a pair of the first trenches in a surface of the semiconductor substrate through an etching process, the first trenches being formed spaced apart from each other, forming oxide in the lower portion of the first trenches, forming poly silicon films on sidewalls of the first trenches, removing the oxide, forming an oxide film by performing thermal oxidation on the semiconductor substrate, and performing planarization on the oxide film. 
     In an embodiment, the forming of the optical modulation part may include forming a first contact layer by implanting impurities into an upper surface of the optical waveguide core layer, forming a depletion layer on the first contact layer by an epitaxial thin film growth process, and forming a second contact layer by implanting impurities into an upper surface of the depletion layer. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying drawings are included to provide a further understanding of the inventive concept, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the inventive concept and, together with the description, serve to explain principles of the inventive concept. In the drawings: 
         FIG. 1  is a plan view of an electro-absorption optical modulation device according to an embodiment of the inventive concept; 
         FIG. 2  is a cross-sectional view taken along line I-I′ in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along line II-II′ in  FIG. 1 ; 
         FIGS. 4 and 5  represent an electro-absorption optical modulation device according to another embodiment of the inventive concept and are cross-sectional views corresponding to line I-I′ in  FIG. 1 ; 
         FIG. 6  is a circuit diagram of an electro-absorption optical modulation device according to an embodiment of the inventive concept; and 
         FIGS. 7 to 13  represent the process of forming an optical waveguide on bulk silicon wafer according to an embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION 
     In order for readers to sufficiently understand the configuration and effect of the inventive concept, exemplary embodiments of the inventive concept are described with reference to the accompanying drawings. The inventive concept may, however, be embodied in different forms and make various changes and should not be construed as limited to the embodiments set forth herein. The embodiments are provided to make the disclosure of the inventive concept complete and completely inform a person skilled in the art of the scope of the inventive concept. A person skilled in the art will be able to understand that the concepts of the inventive concept may be performed in any suitable environments. The same reference numerals throughout the disclosure refer to the same components. 
     The terms used herein are only for explaining embodiments and not intended to limit the inventive concept. The terms in a singular form in the disclosure also includes plural forms unless otherwise specified. The terms used herein “comprises” and/or “comprising” do not exclude the presence or addition of one or more additional components, steps, operations and/or devices other than the components, steps, operations and/or devices that are mentioned. 
     In the present disclosure, when a surface (or layer) is referred to as being ‘on’ another surface (or layer) or substrate, it can be directly on the other surface (or layer) or substrate, or a third surface (or layer) may also be present therebetween. 
     Though terms such as first, second, and third are used to describe various regions and surfaces (or layers) in various embodiments of the present disclosure, the regions and the surfaces are not limited to these terms. These terms are used only to distinguish a certain region or surface (or layer) from another region or surface (or layer). Thus, a surface referred to as a first surface in an embodiment may also be referred to as a second surface in another embodiment. Each embodiment described and illustrated herein also includes its complementary embodiment. Parts indicated by the same reference numerals represent the same components throughout the disclosure. 
     Also, embodiments in the present disclosure are described with reference to ideal, exemplary cross sectional views and/or plan views of the inventive concept. The thicknesses of layers and regions in the drawings are exaggerated for the effective description of technical content. Thus, the forms of exemplary views may vary depending on fabrication technologies and/or tolerances. Thus, embodiments of the inventive concept are not limited to shown specific forms and also include variations in form produced according to fabrication processes. For example, an etched region shown in a rectangular shape may have a round shape or a shape having a certain curvature. Thus, regions illustrated in the drawings are exemplary, and the shapes of the regions illustrated in the drawings are intended to illustrate the specific shapes of the regions of devices and not to limit the scope of the inventive concept. 
     Terms used in embodiments of the inventive concept may be construed as meanings typically known to a person skilled in the art unless being defined otherwise. 
     Exemplary embodiments of the inventive concept are described below in detail with reference to the accompanying drawings. 
     The idea of the inventive concept relates to an electro-absorption optical modulation device. More particularly, it relates to an electro-absorption optical modulation device integrated on a bulk silicon wafer and a method of fabricating the same. 
       FIG. 1  is a plan view of an electro-absorption optical modulation device according to an embodiment of the inventive concept.  FIGS. 2 and 3  are cross-sectional views taken along lines I-I′ and II-II′, respectively in  FIG. 1 .  FIGS. 4 and 5  represent an electro-absorption optical modulation device according to another embodiment of the inventive concept and are cross-sectional views corresponding to line I-I′ in  FIG. 1 . 
     Referring to  FIGS. 1 to 3 , a substrate  100  is provided. The substrate  100  may be a bulk substrate formed of a semiconductor material. For example, the substrate  100  may be a bulk silicon wafer. The substrate  100  may be a single crystal silicon wafer. 
     A first clad layer  110  may be disposed in first trenches  101  and a second trench  103  that are provided on the substrate  100 . The first trenches  101  and the second trench  103  may be extended in a first direction D 1 . The first trenches  101  may be extended from the upper part of the substrate  100  to the inside of the substrate  100  and the second trench  103  may be formed in the substrate  100 . The second trench  103  may connect the first trenches  101 . The first clad layer  110  may include silicon oxide. 
     An optical waveguide core layer  210  may be disposed on the first clad layer  110 . The first clad layer  110  may cover the lower surface and sides of the optical waveguide core layer  210 . The optical waveguide core layer  210  may be disposed spaced apart from the sides  101   a  of the first trenches  101 . The optical waveguide core layer  210  may be extended in the first direction D 1 . The refractive index of the optical waveguide core layer  210  may be higher than that of the first clad layer  110 . For example, the optical waveguide core layer  210  may be single crystal silicon. 
     An optical modulation part  200  may be disposed on the optical waveguide core layer  210 . The optical modulation part  200  may have an island shape. The width of the optical modulation part  200  may be the same as that of the optical waveguide core layer  210 . Referring to  FIGS. 4 and 5 , the width of the second direction D 2  of the optical modulation part  200  may be alternatively smaller or larger than that of the optical waveguide core layer  210 . 
     The optical modulation part  200  may include a first contact layer  201 , a depletion layer  202 , and a second contact layer  203  that are sequentially stacked. The first contact layer  201  may be extended to the outside of the depletion layer  202 . The first contact layer  201  may have a first semiconductor pattern. For example, the first semiconductor pattern may be an n type semiconductor pattern. The second contact layer  203  may have a second semiconductor pattern. For example, the second semiconductor pattern may be a p type semiconductor pattern. The depletion layer  202  may be germanium (Ge) or silicon germanium (SiGe). 
     The second clad layer  120  may be formed on the optical modulation part  200  to cover the optical waveguide core layer  210  and the optical modulation part  200 . For example, the first clad layer  110  and the second clad layer  120  may cover the optical modulation part  200  and the optical waveguide core layer  210 . The second clad layer  120  may have the same material as the first clad layer  110 . For example, the second clad layer  120  may include silicon oxide or silicon nitride. The refractive index of the second clad layer  120  should be smaller than that of the optical waveguide core layer  210 . 
     A first electrode  240   a  may be disposed on the second contact layer  203 . For example, the first electrode  240   a  may be electrically connected to the second contact layer  203 . A second electrode  240   b  may be disposed on the first contact layer  201 . For example, the second electrode  240   b  may be electrically connected to the first contact layer  201 . 
       FIG. 6  is a circuit diagram of an electro-absorption optical modulation device according to an embodiment of the inventive concept. The operating principle of the electro-absorption optical modulation device according to an embodiment is described with reference to  FIG. 6 . An optical signal travelling along the optical waveguide core layer  210  reaches the depletion layer  202  of the optical modulation part  200 . In this case, optical coupling is performed by evanescent coupling. The intensity of the electric field of a reference signal in the depletion layer  202  is adjusted by a voltage applied to the first contact layer  201  and the second contact layer  203  and due to Franz-Keldysh effect, absorptance varies according to the intensity of an electric field applied to the depletion layer  202 . 
     The electro-absorption optical modulation device according to an embodiment of the inventive concept may be fabricated as follows.  FIGS. 7 to 13  represent the process of forming an optical waveguide on bulk silicon wafer according to an embodiment of the inventive concept. 
     Referring to  FIGS. 1 and 7 , it is possible to form a pair of first trenches  101  on a surface of the substrate  100  (e.g., bulk silicon wafer). For example, the first trenches  101  may be formed by the etching of the substrate  100  through an anisotropic etching process using a mask  130 . The first trenches  101  may be formed at an interval in the second direction D 2  intersecting the first direction D 1 . The first trenches  101  may be extended in the first direction D 1 . 
     Referring to  FIG. 8 , it is possible to form oxide  111  in the lower portions of the first trenches  101 . For example, by performing an etching process after the deposition of oxide in the first trenches  101 , it is possible to remain the oxide  111  only in the lower portions of the first trenches  101 . 
     Referring to  FIGS. 9 and 10 , it is possible to form poly silicon  112  on the sidewalls of the first trenches  101 . For example, the poly silicon  112  may be deposited by a chemical vapor deposition (CVD) process and formed by an etching process. By the deposition thickness of the poly silicon  112 , the shapes and sizes of the first clad layer  110  and the optical waveguide core layer  210  may be determined. 
     Referring to  FIG. 11 , it is possible to remove the oxide  111 . For example, it is possible to remove the oxide  111  by performing a wet etching process. 
     Referring to  FIGS. 12 and 13 , it is possible to form an oxide film by performing an oxidation process on the substrate  100 . For example, the oxidation process may be a thermal oxidation process. For example, the second trench  103  connecting the first trenches  101  may be formed in the substrate  100  during the oxidation process. It is possible to perform planarization on the oxide film. Thus, an oxide film that fills the first trenches  101  and the second trench  103  may be formed to work as the first clad layer  110 . The substrate on the first trenches  101  and the second trench  103  may become the optical waveguide core layer  210 . 
     Referring back to  FIGS. 2 and 3 , the optical modulation part  200  is formed on the optical waveguide core layer  210 . It is possible to form the first contact layer  201  by implanting impurities into the upper surface of the optical waveguide core layer  210 . For example, the impurities implanted into the upper surface of the optical waveguide core layer  210  may be an n type semiconductor dopant. The depletion layer  202  is deposited on the first contact layer  201 . For example, it is possible to form the depletion layer  202  on the first contact layer  201  by an epitaxial thin film growth process. It is possible to form the second contact layer  203  by implanting impurities into the upper surface of the depletion layer  202 . For example, the impurities implanted into the upper surface of the depletion layer  202  may be a p type semiconductor dopant. 
     The second clad layer  120  is formed on the optical modulation part  200  and the optical waveguide core layer  210 . For example, the second clad layer  120  may deposit and form silicon oxide by the CVD process. By etching some regions of the second clad layer  120 , openings that expose the first contact layer  201  and the second contact layer  203  are formed. The first electrode  240   a  may be formed by the opening that exposes the first contact layer  201 . The second electrode  240   b  may be formed by the opening that exposes the second contact layer  203 . 
     For example, the first electrode  240   a  may be in electrical contact with the second contact layer  203  through the opening. For example, the second electrode  240   b  may be in electrical contact with the first contact layer through the opening. 
     It is possible to form the electro-absorption optical modulation device on a bulk silicon wafer only with a fundamental semiconductor device fabrication process and a thermal oxide film formation process, without using an expensive SOI wafer. By fabricating an SOI structure locally in a required region only, it is possible to achieve the high integration of various devices on an IC and secure the stability of a structure, without using the expensive SOI wafer. 
     While embodiments of the inventive concept are described with reference to the accompanying drawings, a person skilled in the art may understand that the inventive concept may be practiced in other particular forms without changing technical spirits or essential characteristics. Therefore, embodiments described above should be understood as illustrative and not limitative in every aspect.