Abstract:
Described embodiments include optical connections for electronic-photonic devices, such as optical waveguides and photonic detectors for receiving optical waves from the optical waveguides and directing the optical waves to a common point. Methods of fabricating such connections are also described.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a divisional of U.S. patent application Ser. No. 13/452,064, filed Apr. 20, 2012, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments disclosed herein relate generally to the field of electronic devices (e.g., semiconductor devices) and more particularly to electronic-photonic devices. 
     BACKGROUND 
     Optical transmission may be used as a means for communication between separate integrated circuit chips (inter-chip connections) and within components on the same chip (intra-chip connections). Electronic-photonic devices, also known as optoelectronic devices, are a class of electronic devices that are capable of sourcing, controlling, and/or detecting light. Electronic-photonic devices include both electronic and photonic functions. In response to more demanding communication bandwidth, energy consumption, and performance standards for electronic devices such as semiconductor devices, photonic devices are increasingly being integrated with optical/electrical circuits to form a type of electronic-photonic device called an electronic-photonic integrated circuit. 
     For example, in the semiconductor industry, photonic devices have various applications including communication within a chip, between chips of a computer board, and between computer boards. In chip-to-chip communication via optical interconnects, each chip on the circuit board can be interfaced with a photonic-electronic transmitter-receiver circuit, with two chips operably connected via an optical waveguide. Likewise, optical waveguides may be used to connect components within a chip, such as between an integrated optical source and a photonic detector. Another benefit of electronic-photonic devices is that the elements that perform the pure optical functions, the pure electrical functions and the optoelectronic functions may be formed concurrently, on the same or different substrate, using existing manufacturing processes such as complementary metal oxide semiconductor (CMOS) semiconductor manufacturing processes. 
       FIG. 1  illustrates a block diagram of one example of a conventional electronic-photonic device  100 . Electronic-photonic device  100  may be used to operably connect elements, such as integrated circuits, on a single chip or substrate, or devices on separate substrates. 
     Electronic-photonic device  100  includes a light source  120  configured to generate an optical beam. Light source  120  may be, for example, a coherent light source, such as a laser (such as a hybrid silicon laser or a gallium arsenide laser), a coherent light-emitting diode (LED), a superluminescent diode, or other appropriate light source known in the art. A coherent light source is a light source typically having a narrow wavelength band that is consistent and in-phase. Light source  120  may be configured to output an optical beam having a wavelength in a range of approximately 1,200 nm to 1,550 nm. 
     An optical waveguide  130  connects the optical beam of light source  120  to a modulator  140 , such as an optical ring resonator with a PIN junction. Modulator  140  modulates the received light beam with received electrical data  145 , and outputs the modulated optical data along another waveguide  150 . Modulator  140  is also capable of passing the optical beam through without modulation, such as when the optical beam has already been modulated by another modulator  140  in a same electronic-photonic system. 
     Photonic detector  160  includes a semiconductor material  162  (such as germanium (Ge), silicon germanium (SiGe), indium gallium arsenide (InGaAs), indium phosphate (InP) or other appropriate materials) that is configured to receive and collect the modulated optical beam. The electrical response is transmitted to one or more electrodes  164  that generate an electrical response upon receiving the energy of the wavelengths of the modulated optical data, and provide an external electrical connection for the received optical data. 
       FIGS. 2A and 2B  show cross-sectional views of two examples of optical waveguides  150   a ,  150   b , respectively. Optical waveguides  150   a ,  150   b  both include a respective inner core  152   a ,  152   b  and outer cladding  154   a ,  154   b.    
     Optical waveguide  150   a  ( FIG. 2A ) is an elliptically-shaped optical waveguide. Optical waveguide  150   a  is typical of a waveguide that may be formed as an optical fiber, such as a single mode or multi-mode optical fiber or other element separate from the substrate or chip to which the other photonic devices (e.g., light source  120 , photonic detector  160 , etc.) are formed. Outer core  154   a  may be, for example, a silicon dioxide (SiO 2 ) material Inner core  152   a  may be, for example, a silicon (Si) material, such as SiO 2  doped with impurities such as GeO 2 , and typically has very small dimensions compared to outer cladding  154   a . For example, inner core  152   a  may have a radius of approximately 9 μm, while outer cladding  154   a  may have a radius of approximately 125 μm. 
     Optical waveguide  150   b  ( FIG. 2B ) is a rectangular-shaped waveguide. Optical waveguide  150   b  is typical of an integrated optical waveguide that may be formed on a semiconductor, such as a silicon substrate, a silicon-on-insulator (SOI) substrate, or a printed circuit board (PCB), using lithographic processing. For example, an integrated optical waveguide  150   b  formed on a SiO 2  substrate that acts as the outer cladding  154   b  may have a rectangular inner core  152   b  formed of, for example, a silicon (Si) material. Inner core  152   b  may have a diameter of approximately 300 nm, while outer cladding  154   b  is part of the larger substrate upon which optical waveguide  150   b  is formed and may have a diameter of approximately 1 μm or potentially much larger. 
     Wave guiding of an optical beam through waveguide  150   a ,  150   b  occurs through internal reflection of electromagnetic waves of an optical beam at the interface between the higher refractive index inner core  152   a ,  152   b  and the lower refractive index outer cladding  154   a ,  154   b . Inner core  152   a ,  152   b  is formed of a material with a greater refractive index than the index of the material forming the outer cladding  154   a ,  154   b . The refractive index of inner core  152   a ,  152   b  may be only slightly higher (e.g., 1%) than the refractive index of outer cladding  154   a ,  154   b , or may be significantly higher (referred to as a “high contrast waveguide”) in order to provide greater total internal refraction (TIR). For example, inner core  152   a ,  152   b  may be formed of a silicon (Si) material with a refractive index of approximately 3.5, while outer cladding  154   a ,  154   b  may be formed of a silicon dioxide (SiO 2 ) material with a refractive index of approximately 1.5. 
     It should be understood that outer cladding  154   a ,  154   b  can be formed of any material having a lower refractive index than the index of the inner core  152   a ,  152   b . For example, ambient air, having a refractive index of approximately 1.0, may be used as outer cladding for an optical waveguide  150  having a Si inner core, and thus the cladding need not necessarily use a separate material. It should also be understood that both optical waveguides  130 ,  150  ( FIG. 1 ) may have similar or different characteristics to those described above in connection with  FIGS. 2A and 2B . 
       FIGS. 3A and 3B  illustrate two top-down views of optical connections between an optical waveguide  150  and a photonic detector  160   a ,  160   b .  FIG. 3A  shows a photonic detector  160   a  with optical waveguide  150  butt-coupled to the photonic detector  160   a . Butt-coupled connections for photonic detectors require minimal length for the interconnection. However, the different refractive indexes between optical waveguide  150  and the semiconductor material of photonic detector  160   a  can cause energy from the optical beam to be reflected back into the optical waveguide  150 . For example, optical waveguide  150  may be composed of Si having a refractive index of approximately 1.5, while photonic detector  160   a  may be composed of, e.g., Ge having a refractive index of approximately 4.34. This reflection is known as “return loss,” and in addition to diminishing the strength of the optical signal that is received by photonic detector  160   a , can interfere with operation of light source  120  ( FIG. 1 ). 
       FIG. 3B  shows a photonic detector  160   b  with the optical waveguide  150  evanescent-coupled to the photonic detector  160   b , which is composed of photonic detector portions  160   b   1  and  160   b   2 . Photonic detector portions  160   b   1 ,  160   b   2  surround optical waveguide  150 , but are separated from optical waveguide  150  by distances d 1 , d 2 , respectively. In evanescent coupling, optical waveguide  150  is placed close to photonic detector portions  160   b   1 ,  160   b   2  so that an evanescent field (i.e., a near-field standing wave formed at the boundary between inner core  152   b  and outer cladding  154   b  of  FIG. 5B ) generated by the transmission of the optical beam in optical waveguide  150  reaches photonic detector portions  160   b   1 ,  160   b   2  before fully decaying. Distances d 1 , d 2  must be small enough that the intensity of the evanescent field from optical waveguide  150  does not fully diminish before it is detected by photonic detector portions  160   b   1 ,  160   b   2 . For example, distances d 1 , d 2  may be approximately 10 μm or less. The evanescent field from optical waveguide  150  gives rise to propagating-wave modes on photonic detector portions  160   b   1 ,  160   b   2 , thereby connecting (or coupling) the wave from optical waveguide  150  to photonic detector portions  160   b   1 ,  160   b   2 . 
     Evanescent-coupled photonic detectors  160   b  have lower return loss than butt-coupled photonic detectors  160   a  ( FIG. 3A ), but typically require longer path-lengths (e.g., approximately 50 μm or more) than butt-coupled photonic detectors  160   a . This increases the footprint required for photonic detector  160   b  and thus the overall size of the electronic-photonic device  100  ( FIG. 1 ). 
     Accordingly, it is desirable to provide an optical connection between an optical waveguide and a photonic detector with low return loss yet a small path-length. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a conventional electronic-photonic device. 
         FIGS. 2A and 2B  illustrate cross-sectional diagrams of conventional optical waveguides. 
         FIGS. 3A and 3B  illustrate top-down views of conventional optical connections for an optical waveguide and a photonic detector. 
         FIG. 4  illustrates a top-down view of an optical connection, in accordance with embodiments described herein. 
         FIG. 5  illustrates a top-down view of optical paths in an optical connection, in accordance with embodiments described herein. 
         FIG. 6  illustrates a top-down view of optical paths in another optical connection, in accordance with embodiments described herein. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to various embodiments of the invention. These embodiments are described with sufficient detail to enable those skilled in the art to practice them. It is to be understood that other embodiments may be employed, and that various structural, logical and electrical changes may be made. In addition, where various processes are described, it should be understood that the steps of the processes may occur in an order other than how they are specifically described, unless noted otherwise. 
     Embodiments described herein make advantageous use of a phenomenon known as bending loss that occurs with optical waveguides. When an optical beam travels in an optical waveguide, a near-field standing wave, referred to as an evanescent wave, is formed at the boundary between the inner core and outer cladding of the optical waveguide. When a bend occurs in the optical waveguide, the portion of the evanescent wave located outside of the border between the inner core and the outer cladding must travel faster than the portion of the wave located inside of the inner core, in order to maintain the same angular velocity. At a point referred to as the “critical radius,” the evanescent wave cannot travel fast enough in the respective medium to maintain the same angular velocity as the portion of the wave inside of the waveguide, and the energy of this portion propagates outward from the waveguide in a radial direction away from the curved waveguide. 
     Bending loss is typically considered an obstacle in optical waveguide design. The embodiments described below, however, exacerbate and take advantage of this phenomenon to provide a connection between an optical waveguide and a photonic detector. 
       FIG. 4  illustrates a top-down view of an optical connection  400  between an optical waveguide  410  and a photonic detector  420 . Optical connection  400  may be formed, for example, on a substrate such as a silicon substrate, a silicon-on-insulator (SOI) substrate, a silicon dioxide (SiO 2 ) substrate, or a printed circuit board (PCB). Alternatively, elements of optical connection  400  may be formed on multiple separate substrates (e.g., Si, SiO 2 , SOI, or other suitable substrates). 
     Optical waveguide  410  includes an internal core  412  and outer cladding  414 , and may be integrated into a substrate (e.g., a common substrate with photonic detector  420 ), or may be, e.g., a single mode or dual mode optical fiber. Inner core  412  may be formed of, for example, a Si material, and have a width of approximately 300 nm Outer cladding  414  may be formed of, for example, SiO 2 . Inner core  412  may be patterned in outer cladding  414  using known processes. 
     Photonic detector  420  is composed of a semiconductor material, such as germanium (Ge), silicon germanium (SiGe), indium gallium arsenide (InGaAs), indium phosphate (InP) or other appropriate materials, that generates an electrical response upon receiving an optical wave from optical waveguide  410 , as described below. Photonic detector  420  includes at least one electrode  430 , which may be composed of a metal such as aluminum, copper, or titanium, for example. Photonic detector  420  may be fabricated using wafer bonding and other existing manufacturing processes such as complementary metal oxide semiconductor (CMOS) semiconductor manufacturing processes. 
     The operably connected end of optical waveguide  410  is curved at an angle of θ 1 , with a corresponding radius of curvature r 1 . The curved portion of optical waveguide  410  may be formed using a lithographic process. Radius of curvature r 1  may be constant along the curve of optical waveguide  410 , or alternatively may vary as a function of angle θ 1 . If radius of curvature r 1  is sufficiently small (i.e., is equal to or less than the “critical radius”), thereby creating a sharp enough curve in optical waveguide  410 , the evanescent wave from optical waveguide  410  leaves optical waveguide  410  and propagates radially towards photonic detector  420 . The critical radius of waveguide  410  will depend on the width of the inner core  412 , and the materials and respective refraction indexes for the inner core  412  and the outer cladding  414 . For an optical waveguide  410  including, for example, a 300 nm wide Si inner core  412  and a SiO 2  outer cladding, radius r 1  may be equal to or less than 1 μm. 
       FIG. 5  illustrates a top-down view of the paths of evanescent waves propagating radially from optical waveguide  410  to photonic detector  420  in optical connection  400 . 
     The semiconductor material used to form photonic detector  420  may be shaped to reflect the propagated evanescent waves to a common point (e.g., electrode  430 ). The semiconductor material may be shaped using, for example, a lithographic process, such as electron-beam lithography, or through etching techniques. The reflecting edge  425  of photonic detector  420  is preferably in a range of about 5-15 μm from optical waveguide  410 , providing an adequate path length for the wavelengths of the propagated evanescent waves while allowing for a compact photonic detector  420 . 
     The radially propagated evanescent waves received by photonic detector  420  can be reflected at a substantially uniform angle θ 2  from edge  425  towards electrode  430 . For example, the reflecting edge  425  of photonic detector  420  may be shaped to reflect the evanescent waves at approximately a 20° angle towards electrode  430 . In other embodiments, angle θ 2  may change as a function of its distance from optical waveguide  410 . Selecting a common point for electrode  430  that is relatively close (i.e., within 10 μm) to optical waveguide  410  allows for a smoother reflecting edge  425 , because complicated reflection points are not required. 
     In order to better promote reflectivity of the optical beams, the photonic detector  420  may be formed of a material having a higher refractive index than the surrounding substrate. For example, a germanium (Ge) photonic detector  420  having an index of refraction of approximately 4.34 may be used in a substrate of SiO 2 , which has a refractive index of approximately 1.5. Other materials may also be used to form the photonic detector  420 , such as InP, SiGe, GaAs, and other appropriate materials. 
       FIG. 6  illustrates a top-down view of optical paths in an optical connection  500  in another embodiment of a photonic detector  520 . Photonic detector  520  includes a shaped reflecting edge  525  similar to edge  425  described above in connection with  FIG. 4 , in order to reflect evanescent waves propagated radially from optical waveguide  410  due to bending loss. In addition, a bottom portion of photonic detector  520  is butt-coupled to the terminal point  418  of optical waveguide  410 , in order to couple any remaining optical beam from inner core  414  to photonic detector  520 . The optical beam is reflected from another reflecting edge  527  of photonic detector  520 , and may be reflected several times prior to reaching the common point (e.g., electrode  430 ). Photonic detector  520  may be fabricated using wafer bonding and other existing manufacturing processes such as complementary metal oxide semiconductor (CMOS) semiconductor manufacturing processes. 
     Optical connections including an optical waveguide  410  and/or a photonic detector  420 ,  520  as described in connection with  FIGS. 4-6  may be used in various electronic-photonic devices. For example, the optical connections could be used with inter-chip or intra-chip systems including at least one light source  120  and modulator  140  ( FIG. 1 ), such as to connect multiple memory elements (e.g., one or more cores or DRAM, SDRAM, SRAM, ROM, or other type of solid-state or static memory elements). 
     The above description and drawings are only to be considered illustrative of specific embodiments, which achieve the features and advantages described herein. Modification and substitutions to specific processes, components, and structures can be made. For example, it should be understood that appropriate types of semiconductor materials and memory elements other than those specifically described in connection with the above embodiments may be used. Accordingly, the embodiments of the invention are not to be considered as being limited by the foregoing description and drawings, but only by the scope of the appended claims.