Method and apparatus providing a coupled photonic structure

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.

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. 1illustrates a block diagram of one example of a conventional electronic-photonic device100. Electronic-photonic device100may be used to operably connect elements, such as integrated circuits, on a single chip or substrate, or devices on separate substrates.

Electronic-photonic device100includes a light source120configured to generate an optical beam. Light source120may 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 source120may be configured to output an optical beam having a wavelength in a range of approximately 1,200 nm to 1,550 nm.

An optical waveguide130connects the optical beam of light source120to a modulator140, such as an optical ring resonator with a PIN junction. Modulator140modulates the received light beam with received electrical data145, and outputs the modulated optical data along another waveguide150. Modulator140is also capable of passing the optical beam through without modulation, such as when the optical beam has already been modulated by another modulator140in a same electronic-photonic system.

Photonic detector160includes a semiconductor material162(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 electrodes164that 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 2Bshow cross-sectional views of two examples of optical waveguides150a,150b, respectively. Optical waveguides150a,150bboth include a respective inner core152a,152band outer cladding154a,154b.

Optical waveguide150a(FIG. 2A) is an elliptically-shaped optical waveguide. Optical waveguide150ais 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 source120, photonic detector160, etc.) are formed. Outer core154amay be, for example, a silicon dioxide (SiO2) material. Inner core152amay be, for example, a silicon (Si) material, such as SiO2doped with impurities such as GeO2, and typically has very small dimensions compared to outer cladding154a. For example, inner core152amay have a radius of approximately 9 μm, while outer cladding154amay have a radius of approximately 125 μm.

Optical waveguide150b(FIG. 2B) is a rectangular-shaped waveguide. Optical waveguide150bis 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 waveguide150bformed on a SiO2substrate that acts as the outer cladding154bmay have a rectangular inner core152bformed of, for example, a silicon (Si) material. Inner core152bmay have a diameter of approximately 300 nm, while outer cladding154bis part of the larger substrate upon which optical waveguide150bis formed and may have a diameter of approximately 1 μm or potentially much larger.

Wave guiding of an optical beam through waveguide150a,150boccurs through internal reflection of electromagnetic waves of an optical beam at the interface between the higher refractive index inner core152a,152band the lower refractive index outer cladding154a,154b. Inner core152a,152bis formed of a material with a greater refractive index than the index of the material forming the outer cladding154a,154b. The refractive index of inner core152a,152bmay be only slightly higher (e.g., 1%) than the refractive index of outer cladding154a,154b, or may be significantly higher (referred to as a “high contrast waveguide”) in order to provide greater total internal refraction (TIR). For example, inner core152a,152bmay be formed of a silicon (Si) material with a refractive index of approximately 3.5, while outer cladding154a,154bmay be formed of a silicon dioxide (SiO2) material with a refractive index of approximately 1.5.

It should be understood that outer cladding154a,154bcan be formed of any material having a lower refractive index than the index of the inner core152a,152b. For example, ambient air, having a refractive index of approximately 1.0, may be used as outer cladding for an optical waveguide150having a Si inner core, and thus the cladding need not necessarily use a separate material. It should also be understood that both optical waveguides130,150(FIG. 1) may have similar or different characteristics to those described above in connection withFIGS. 2A and 2B.

FIGS. 3A and 3Billustrate two top-down views of optical connections between an optical waveguide150and a photonic detector160a,160b.FIG. 3Ashows a photonic detector160awith optical waveguide150butt-coupled to the photonic detector160a. Butt-coupled connections for photonic detectors require minimal length for the interconnection. However, the different refractive indexes between optical waveguide150and the semiconductor material of photonic detector160acan cause energy from the optical beam to be reflected back into the optical waveguide150. For example, optical waveguide150may be composed of Si having a refractive index of approximately 1.5, while photonic detector160amay 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 detector160a, can interfere with operation of light source120(FIG. 1).

FIG. 3Bshows a photonic detector160bwith the optical waveguide150evanescent-coupled to the photonic detector160b, which is composed of photonic detector portions160b1and160b2. Photonic detector portions160b1,160b2surround optical waveguide150, but are separated from optical waveguide150by distances d1, d2, respectively. In evanescent coupling, optical waveguide150is placed close to photonic detector portions160b1,160b2so that an evanescent field (i.e., a near-field standing wave formed at the boundary between inner core152band outer cladding154bofFIG. 5B) generated by the transmission of the optical beam in optical waveguide150reaches photonic detector portions160b1,160b2before fully decaying. Distances d1, d2must be small enough that the intensity of the evanescent field from optical waveguide150does not fully diminish before it is detected by photonic detector portions160b1,160b2. For example, distances d1, d2may be approximately 10 μm or less. The evanescent field from optical waveguide150gives rise to propagating-wave modes on photonic detector portions160b1,160b2, thereby connecting (or coupling) the wave from optical waveguide150to photonic detector portions160b1,160b2.

Evanescent-coupled photonic detectors160bhave lower return loss than butt-coupled photonic detectors160a(FIG. 3A), but typically require longer path-lengths (e.g., approximately 50 μm or more) than butt-coupled photonic detectors160a. This increases the footprint required for photonic detector160band thus the overall size of the electronic-photonic device100(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.

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. 4illustrates a top-down view of an optical connection400between an optical waveguide410and a photonic detector420. Optical connection400may be formed, for example, on a substrate such as a silicon substrate, a silicon-on-insulator (SOI) substrate, a silicon dioxide (SiO2) substrate, or a printed circuit board (PCB). Alternatively, elements of optical connection400may be formed on multiple separate substrates (e.g., Si, SiO2, SOI, or other suitable substrates).

Optical waveguide410includes an internal core412and outer cladding414, and may be integrated into a substrate (e.g., a common substrate with photonic detector420), or may be, e.g., a single mode or dual mode optical fiber. Inner core412may be formed of, for example, a Si material, and have a width of approximately 300 nm. Outer cladding414may be formed of, for example, SiO2. Inner core412may be patterned in outer cladding414using known processes.

Photonic detector420is 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 waveguide410, as described below. Photonic detector420includes at least one electrode430, which may be composed of a metal such as aluminum, copper, or titanium, for example. Photonic detector420may 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 waveguide410is curved at an angle of θ1, with a corresponding radius of curvature r1. The curved portion of optical waveguide410may be formed using a lithographic process. Radius of curvature r1may be constant along the curve of optical waveguide410, or alternatively may vary as a function of angle θ1. If radius of curvature r1is sufficiently small (i.e., is equal to or less than the “critical radius”), thereby creating a sharp enough curve in optical waveguide410, the evanescent wave from optical waveguide410leaves optical waveguide410and propagates radially towards photonic detector420. The critical radius of waveguide410will depend on the width of the inner core412, and the materials and respective refraction indexes for the inner core412and the outer cladding414. For an optical waveguide410including, for example, a 300 nm wide Si inner core412and a SiO2outer cladding, radius r1may be equal to or less than 1 μm.

FIG. 5illustrates a top-down view of the paths of evanescent waves propagating radially from optical waveguide410to photonic detector420in optical connection400.

The semiconductor material used to form photonic detector420may be shaped to reflect the propagated evanescent waves to a common point (e.g., electrode430). The semiconductor material may be shaped using, for example, a lithographic process, such as electron-beam lithography, or through etching techniques. The reflecting edge425of photonic detector420is preferably in a range of about 5-15 μm from optical waveguide410, providing an adequate path length for the wavelengths of the propagated evanescent waves while allowing for a compact photonic detector420.

The radially propagated evanescent waves received by photonic detector420can be reflected at a substantially uniform angle θ2from edge425towards electrode430. For example, the reflecting edge425of photonic detector420may be shaped to reflect the evanescent waves at approximately a 20° angle towards electrode430. In other embodiments, angle θ2may change as a function of its distance from optical waveguide410. Selecting a common point for electrode430that is relatively close (i.e., within 10 μm) to optical waveguide410allows for a smoother reflecting edge425, because complicated reflection points are not required.

In order to better promote reflectivity of the optical beams, the photonic detector420may be formed of a material having a higher refractive index than the surrounding substrate. For example, a germanium (Ge) photonic detector420having an index of refraction of approximately 4.34 may be used in a substrate of SiO2, which has a refractive index of approximately 1.5. Other materials may also be used to form the photonic detector420, such as InP, SiGe, GaAs, and other appropriate materials.

FIG. 6illustrates a top-down view of optical paths in an optical connection500in another embodiment of a photonic detector520. Photonic detector520includes a shaped reflecting edge525similar to edge425described above in connection withFIG. 4, in order to reflect evanescent waves propagated radially from optical waveguide410due to bending loss. In addition, a bottom portion of photonic detector520is butt-coupled to the terminal point418of optical waveguide410, in order to couple any remaining optical beam from inner core414to photonic detector520. The optical beam is reflected from another reflecting edge527of photonic detector520, and may be reflected several times prior to reaching the common point (e.g., electrode430). Photonic detector520may 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 waveguide410and/or a photonic detector420,520as described in connection withFIGS. 4-6may 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 source120and modulator140(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.