Patent Publication Number: US-2007101927-A1

Title: Silicon based optical waveguide structures and methods of manufacture

Description:
TECHNICAL FIELD  
      The present invention relates to optical interconnections for silicon based photonic integrated circuits. More particularly, the present invention relates to silicon based thin-film waveguide structures for providing optical communication between components of photonic integrated circuits and methods of making such structures.  
     BACKGROUND  
      Photonic integrated circuits provide an integrated platform increasingly used to form complex optical systems. This technology allows many optical devices, both active and passive, to be integrated on a single substrate. For example, photonic integrated circuits may comprise integrated lasers, integrated receivers, waveguides, detectors, semiconductor optical amplifiers, and other active and passive semiconductor optical devices. Such monolithic integration of active and passive devices provides an effective integrated technology platform for use in optical communications, information processing and storage and the like.  
      Photonic integrated circuits rely on efficient optical interconnections to transmit light between the components and devices that form these integrated circuits. Conventional optical interconnections usually employ thin-film optical waveguides as device interconnects. Specifically, circuit fabricators have used thin-films of semiconductor materials to form optical waveguides that are integrated with thin-film optical, electronic, and opto-electronic devices formed on the substrate of the photonic integrated circuit. When a light-transmissive material is surrounded or otherwise bounded by another material having a lower refractive index, light propagating through the inner material is reflected at the boundary between the two materials. This produces a guiding effect. However, light can be lost at this boundary because of edge effects, surface imperfections, roughness, and the like. In this regard, it is desired that optical-propagation losses be kept to a minimum in such waveguides to provide efficient photonic integrated circuits.  
      In conventional methods, optical waveguides are generally formed on a substrate by photolithography. One type of optical waveguide is known as a ridge waveguide. Ridge waveguides are typically made by masking a portion of the substrate and etching away or otherwise removing an exposed portion to define guiding sidewalls of the optical waveguide. As a result, the cross section of the waveguide is normally square or trapezoidal in shape. When the light transmitting material of a waveguide is fabricated by etching in this way, its side surfaces can be roughened, and hence, undesirable transmission loss can occur.  
     SUMMARY  
      The present invention thus provides methods of making silicon based thin-film optical waveguides with minimal optical-propagation losses. In particular, optical waveguides in accordance with the present invention can be formed without the need to etch sidewalls of the light guiding material of the waveguide. In this way, optical transmission losses caused by surface imperfections or roughness can be minimized or eliminated. Moreover, the present invention provides a way to integrate a silicon waveguide with one or more optical, electronic, or opto-electronic devices on a common substrate.  
      Optical waveguides in accordance with the present invention can be used in photonic integrated circuits for providing functions, such as optical transmission, optical branching/combining, wavelength filtering, wavelength multiplexing or demultiplexing, and optical modulation of light intensity or phase. Such waveguides can be used in the fields of optical information transmission, such as optical communication and optical interconnection, and information processing, such as optical memory.  
      Accordingly, in one aspect of the present invention a method of making a silicon based thin-film optical waveguide is provided. The method generally comprises the steps of providing a substrate, depositing a thin-film dielectric layer on the substrate, forming a channel in the thin-film dielectric layer, and providing a silicon layer in the channel. The substrate comprises a silicon layer having a surface. The thin-film dielectric layer is deposited on at least a portion of the surface of the silicon layer of the substrate. The channel in the thin-film dielectric layer exposes a portion of the surface of the silicon layer of the substrate, which defines at least a portion of a path for an optical waveguide. The silicon layer provided in the channel is in contact with the exposed portion of the surface of the silicon layer of the substrate.  
      In another aspect of the present invention, a method of making a silicon based thin-film optical waveguide that is integrated with a silicon-on-insulator substrate is provided. The method generally comprises the steps of providing a silicon-on-insulator substrate, depositing a thin-film dielectric layer on the substrate, forming a channel in the thin-film dielectric layer, and providing a single crystal silicon layer in the channel. The substrate comprises a silicon-on-insulator substrate having a single crystal silicon layer having a surface. The thin-film dielectric layer is deposited on at least a portion of the surface of the single crystal silicon layer of the substrate. The channel in the thin-film dielectric layer exposes a portion of the surface of the single crystal silicon layer of the substrate, which defines at least a portion of a path for an optical waveguide. The single crystal silicon layer provided in the channel is in contact with the exposed portion of the surface of the single crystal silicon layer of the silicon-on-insulator substrate.  
      In yet another aspect of the present invention, a method of making a silicon based photonic integrated circuit is provided. Generally, the method comprises the steps of providing a silicon-on-insulator substrate, depositing a thin-film dielectric layer on the substrate, forming a thin-film optical waveguide, and forming an opto-electronic device. The substrate comprises a silicon-on-insulator substrate having a single crystal silicon layer having a surface. The thin-film dielectric layer is deposited on at least a portion of the surface of the single crystal silicon layer of the substrate. The thin-film optical waveguide is provided by first forming a channel in the thin-film dielectric layer that exposes a portion of the surface of the single crystal silicon layer, which channel defines at least a portion of a path for the optical waveguide and subsequently providing a single crystal silicon layer in at least a portion of the channel. The opto-electronic device is formed in at least a portion of the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:  
       FIG. 1  is a schematic cross-sectional view of an exemplary optical waveguide of the present invention showing in particular first and second silicon layers that define a guiding region of the waveguide;  
       FIG. 2  is a schematic cross-sectional view of a silicon-on-insulator structure that can be used to form an optical waveguide in accordance with the present invention;  
       FIG. 3  is a schematic cross-sectional view of the silicon-on-insulator structure of  FIG. 2  showing in particular a thin-film dielectric layer to provide a layered structure having a channel for defining a waveguide path in accordance with the present invention;  
       FIG. 4  is a schematic cross-sectional view of the layered structure of  FIG. 3  showing in particular a silicon layer that has been provided on the layered structure and in the channel to define a guiding portion of a waveguide; and  
       FIG. 5  is a schematic cross-sectional view of the layered structure of  FIG. 4  after partial removal of the silicon layer, thus providing a planarized structure that provides an optical waveguide in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION  
      In  FIG. 1 , an optical waveguide  10  in accordance with the present invention is schematically illustrated in cross-section. As shown, the optical waveguide  10  includes a substrate  12 , preferably silicon, buried oxide layer  14 , a first silicon layer  16  having silicon surface  18 , second silicon layer  20  having surface  22 , and dielectric layer  24 . The first and second silicon layers,  16  and  20 , function as the light transmissive material through which light travels in a propagation direction. First and second interfaces,  26  and  28 , between the dielectric layer  24  and the second silicon layer  20  as well as the surface  22  of the second silicon layer  20  function to confine and guide light in a guiding region  30  when the index of refraction of the dielectric layer  24  is less than that of the silicon layer,  16  and  20 . The dielectric layer  24  may be formed from or include materials, or combination thereof, such as silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, as well as those materials generally characterized as dielectrics or insulators. The surface  22  of second silicon layer  20  can similarly provide a guiding function when the surface forms an interface with ambient air or other gas having an index of refraction lower than that of silicon. If desired, a thin-film layer (not shown) can be provided on the surface  22  for any guiding, passivating, or protective functions, or the like. Also, other functional layers such as passivating or protective layers, for example, can be provided anywhere in the layered structure of the waveguide  10 .  
      In order to illustrate such guiding and confining functionality, a mode  32  of an electromagnetic field that can propagate through the guiding region  30  of the optical waveguide  10  is illustrated schematically. More specifically, the waveguide  10  is preferably designed for single mode transmission. That is, the waveguide  10  is preferably designed so that the lowest order bound mode (also called the fundamental guided mode or trapped mode) can propagate at the wavelength of interest. For typical optical communications systems, wavelengths in the near infra-red portion of the electromagnetic spectrum are typically used. For example, wavelengths around 1.55 microns are common.  
      Preferably, the first silicon layer  16  and the buried oxide layer  14  are provided as a silicon-on-insulator structure, as such are conventionally known or as may be further advanced in the future. However, the optical waveguide  10  does not require use of silicon-on-insulator technology and the layered thin-film structure of the optical waveguide  10  may be formed by any appropriate thin-film deposition and processing techniques. Silicon-on-insulator structures are preferred because of their compatibility with conventional complementary metal oxide semiconductor (CMOS) processing. Silicon-on-insulator structures are also preferred because such structure typically provides high quality single crystal silicon material as the first silicon layer  14 . Such single crystal silicon material generally has minimal defects or imperfections that can contribute to optical losses. Also, the optical functionality of photonics based devices such as optical modulators, laser, and switches, and the like can be integrated with the electrical functionality of devices such as transistors, resistors, capacitors, and inductors on the same substrate. These opto-electronic and electronic devices can be formed by using the common processing techniques to provide optical or photonic circuits that are integrated with electronic circuits and devices. Moreover, silicon-on-insulator technology provides an easy way to provide a high quality single crystal layer and to electrically isolate plural devices that can be formed in the silicon layer from each other.  
      Optical waveguides in accordance with the present invention, such as the optical waveguide  10  shown in  FIG. 1 , can be made as described below. Preferably, conventional CMOS processing techniques can be used although any other known or developed techniques can be used instead of or in combination.  
      Referring to  FIG. 2 , a typical silicon-on-insulator structure  34  is illustrated that includes substrate  36  (conventionally a silicon layer), dielectric layer  38  (conventionally known as a buried oxide layer), and first silicon layer  40  having surface  42  is illustrated. Such silicon-on-insulator substrates are commercially available and it is contemplated that similarly functioning future developed structures can be used for making waveguide structures and photonic integrated circuits in accordance with the present invention. The thickness of the buried oxide layer  38  and the thickness of the first silicon layer  40  are preferably selected by considering certain desired properties of the particular optical waveguide or photonic integrated circuit to be made, such as the dimensions and/or structure of the other devices or components of the photonic integrated circuit, the ability to create and isolate devices and components, as well as the processing techniques to be used.  
      Preferably, as shown in  FIG. 3 , a dielectric layer  44  having a channel  46  formed therein is provided on the surface  42  of the first silicon layer  40 . The dielectric layer  44  may be silicon dioxide or silicon nitride, for example. As shown, the channel  46  exposes a surface portion  48  of the surface  42  of the first silicon layer  40 . The channel  46  also functions to define a path or route for the waveguide. The dielectric layer  44  and channel  46  can be formed by using any appropriate conventionally known or future developed deposition, photolithography, and/or etching techniques. For example, the dielectric layer  44  can be deposited on the surface  42  as a blanket thin-film, masked to define the channel  46 , and etched to remove a portion of the dielectric layer  44  to form the channel  46 . As another example, deposition of the dielectric layer  44  can be controlled selectively prevent deposition of dielectric in the region of the channel  46  during the deposition step. That is, a mask can be provided on the surface  42  of the silicon layer  40  where it is desired to form the channel  46 . Dielectric material can be deposited on the masked and unmasked surfaces and a liftoff technique can be used to remove the mask together with deposited material in the region of the channel  46 . In this regard, wet and/or dry etching techniques are contemplated. Any deposition techniques may be used such as those including chemical vapor deposition, physical vapor deposition, and the like.  
      After the channel is created, a second silicon layer  50  is provided such as shown in  FIG. 4 . As illustrated, the second silicon layer  50  includes a waveguide portion  52  in the channel  46  and an overcoat portion  54  that is on the dielectric layer  44  and over the portion  52 . In accordance with the present invention, the silicon layer  50  can be epitaxially grown as a crystalline thin film or deposited as an amorphous or partially crystalline film and subsequently at least partially or further crystallized. For example, the silicon layer  50  can by provided by using a deposition and/or crystallization process as described in commonly owned co-pending U.S. Patent Application having Attorney Docket No. HON0012/US, entitled SILICON-INSULATOR-SILICON THIN-FILM STRUCTURES FOR OPTICAL MODULATORS AND METHODS OF MANUFACTURE, filed on Aug. 10, 2004 and having Ser. No. 10/915,299, the entire disclosure of which is fully incorporated herein by reference for all purposes.  
      With respect to epitaxially growing the second silicon layer  50 , the surface  48  preferably functions as a seed or template to initiate crystal growth in accordance with the present invention. Growth of epitaxial material preferably originates at or from surface  48 . In this way, vacuum deposition processes such as molecular beam epitaxy or metal organic chemical vapor deposition or the like can be used to grow a crystalline silicon layer on the surface  48 . The second silicon layer  50  can be provided in a way that allows formation of the overcoat portion  54  or in a way that prevents formation of the overcoat portion  54  such as by using a masking technique as noted below. In any event, a crystalline silicon layer is preferably epitaxially provided in the channel  46  in accordance with the present invention.  
      In accordance with the present invention an amorphous silicon layer can be deposited in the channel  46  to provide the waveguide portion  52  and overcoat portion  54 . Any technique such as low pressure chemical vapor deposition or the like, for example, can be used. The waveguide portion  52  of the silicon layer  50 , if provided as an amorphous or polycrystalline material, is preferably thermally processed such as by using a furnace, epi reactor, rapid thermal processor, heated element, or laser system to at least partially crystallize the waveguide portion  52  of the second silicon layer  50 . The surface  48  can also function to help crystallize such an amorphous silicon layer when the waveguide portion  52  is provided this way.  
      Any process can be used that is capable of at least partially crystallizing a silicon layer, such as an amorphous silicon layer, to provide a desired material quality. Such crystallization can be done at any time after the second silicon layer is formed. Moreover, any process capable of improving the optical transmission properties of a silicon material, whether crystalline or not, may be used. Moreover, such a technique can be used to improve the crystallinity, such as by reducing defects or the like, of a crystalline, polycrystalline or partially crystalline silicon layer for the purpose of improving optical transmission properties. For example, crystallization of deposited silicon films by furnace, lamp, and laser techniques at a sufficient temperature and time to achieve a desired degree of crystallization can be used.  
      At any time after deposition of the second silicon layer  50 , any desired portion of the overcoat portion  54  can be substantially or partially removed to define a waveguide structure  56  as illustrated in  FIG. 5 . However, it is contemplated that the overcoat portion  54  does not need to be removed. Removal can be done, for example, before or after an amorphous material is crystallized or partially crystallized. For example, a wet or dry etching technique can be used. Also, a planarization process can be used, such as by using chemical mechanical processing (CMP). Any known or developed methods for planarizing or removing materials are contemplated, and such processes may be conducted by any number of combined steps of multiple varieties.  
      It is contemplated that the waveguide structure  56  can be made without forming an overcoat portion  54  of the second silicon layer  50  by providing silicon only within the defined channel. Conventionally known or future developed photolithography and/or masking techniques can be used to limit or prevent material from being deposited in certain predetermined regions. For example, a mask formed from photosensitive material can be used to prevent deposition on the dielectric layer  44 . If desired, the same mask that is used to define and form the channel  46  can be used along with an appropriate liftoff process. It is also contemplated that techniques such as selective epitaxial growth can be used in accordance with the present invention.  
      In accordance with the present invention, the waveguide structure  56  can be used to provide one or more optical interconnection between any desired opto-electronic devices or electrical devices of a photonic integrated circuit or the like. Such opto-electronic device may include lasers, receivers, detectors, semiconductor optical amplifiers, and other active and passive semiconductor optical devices. Such electronic devices may include transistors, resistors, capacitors, and inductors. A waveguide in accordance with the present invention may provide any desired optical interconnection or communication path between such devices or components including paths that split or combine optical signals.  
      The waveguide structure  56  is particularly advantageous because the first silicon layer  40  and the waveguide portion  52  of the second silicon layer  50  can be provided as high quality material that can provide low transmission loss optical communication. Preferably, the silicon layer  40  comprises a single crystal silicon layer that has minimal crystal defects or imperfections that could contribute to optical transmission losses. Such high quality silicon material is available from preferred silicon-on-insulator structures but may be formed from other suitable growth techniques. Because the waveguide portion  52  of the second silicon layer  50  can be epitaxial grown or crystallized from surface  48  of the silicon layer  40  in accordance with the present invention, a single crystal silicon layer having minimal crystal defects can be provided as the waveguide portion  52 . In this way, the waveguide portion  52  effectively functions as an extension of the silicon layer  40 . The combination of the waveguide portion  52  and the silicon layer provides a guiding region with greater cross-sectional area than can be provided by the silicon layer  40  alone. This can be particularly advantageous where the thickness of silicon layer  40  is limited, such as based on the structure or design of any opto-electronic or electronic devices integrated on the same substrate as the waveguide  56 .  
      The present invention has now been described with reference to several embodiments thereof. The entire disclosure of any patent or patent application identified herein is hereby incorporated by reference. The foregoing detailed description and examples have been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. It will be apparent to those skilled in the art that many changes can be made in the embodiments described without departing from the scope of the invention. Thus, the scope of the present invention should not be limited to the structures described herein, but only by the structures described by the language of the claims and the equivalents of those structures.