Patent Publication Number: US-6987912-B2

Title: Epitaxial growth for waveguide tapering

Description:
This application is a Divisional of 10/159,238 filed May 31, 2002. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of, and claims priority under 35 U.S.C. § 120 from, U.S. patent application Ser. No. 10/159,238, filed May 31, 2002, and still pending, which is related to commonly-assigned and co-filed U.S. patent application Ser. No. 10/160,625, filed May 31, 2002, entitled “Method For Producing Vertical Tapers In Optical Waveguides By Over Polishing” by M. Salib, and to U.S. patent application Ser. No. 10/159,379, filed May 31, 2002, entitled “Fabrication Of A Waveguide Taper Through Ion Implantation” by M. Salib et al. 
    
    
     FIELD OF THE INVENTION 
     The field of invention relates to optical communication devices in general; and, more specifically but not limited to waveguide tapers in optical devices. 
     BACKGROUND 
     Some optical devices may include a waveguide that is intended to be coupled to another waveguide or fiber having a significantly larger cross-sectional size. For example, a planar lightwave circuit (PLC) can have a waveguide on the order of four microns in width to be coupled an optical fiber with a diameter of about ten microns. One way to couple a port of a relatively large waveguide to a port of a significantly smaller waveguide is by forming a tapered waveguide structure to couple the two waveguides. In one type of taper, the taper at one end has a width or diameter of about the same size as the larger waveguide. At the other end, the taper comes to a point. The sides of the taper are typically straight so that the taper has a wedge-like shape, with the taper narrowing from the wide end to the point or narrow end. The wide end of the taper is used to couple the taper from the larger waveguide. The idea behind this taper is to create a virtual, vertical effective index change in the waveguide that forces the mode into an underlying, single-mode waveguide. As the taper becomes narrower, the effective index decreases, and the mode moves lower in the semiconductor material. 
     One conventional technique to form the above-described taper when the smaller waveguide is a semiconductor waveguide is to etch one end of the smaller waveguide to form the taper. For example, at the end of the waveguide, the smaller waveguide has: (a) a length about equal to the desired length of the taper; and (b) a thickness that is about equal to the sum of the desired thickness of the smaller waveguide and the desired thickness of the taper. For example, the resulting thickness can be about the height of the core of an optical fiber. This end of the smaller waveguide is then etched using standard etching techniques to form the taper with a shape as described above. However, some etching processes form the taper&#39;s point so that it appears eroded, instead of the desired sharp edge or point. This erosion can degrade performance of the taper. In addition, typical etching processes cause the etched surfaces to be significantly less smooth than the surfaces that are not etched. This roughness can increase the waveguide&#39;s loss (e.g., in some tests the etched surfaces increased loss an addition five to ten decibels). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts or elements having the same or substantially similar functions and/or structures throughout the various views unless otherwise specified. Further, terms such as “top”, “upper”, “lower”, “vertical”, “lateral”, “beneath”, etc. may be used herein in describing the figures. These terms are used in a relative sense to show relative orientation of the parts or elements as depicted in the figures and not necessarily with respect gravity or as physical embodiments may be oriented during use. 
         FIGS. 1 and 1A  are representative cross-sectional and top views of an initial stage in fabricating a taper, according to one embodiment of the present invention. 
         FIGS. 2 and 2A  are representative cross-sectional and top views of another stage in fabricating a taper, respectively, according to one embodiment of the present invention. 
         FIGS. 3 and 3A  are representative cross-sectional and top views of still another stage in fabricating a taper, respectively, according to one embodiment of the present invention. 
         FIGS. 4 and 4A  are representative cross-sectional and top views of yet another stage in fabricating a taper, respectively, according to one embodiment of the present invention. 
         FIG. 5  is a representative isometric perspective view of a section cut as indicated in  FIG. 4 , according to an embodiment of the present invention. 
         FIG. 6  is a block diagram illustrating an exemplary system using a taper fabricating according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a partial cross-section of a semiconductor workpiece (not to scale) during an early stage in fabricating a taper, according to one embodiment of the present invention. The workpiece includes a semiconductor substrate  10 , an insulator layer  12 , a silicon layer  14 , and a protective layer  16 . Silicon layer  14  is formed so as to serve as a waveguide. In one embodiment, silicon layer  14  is formed so as to serve as a rib waveguide. 
     More particularly, insulating layer  12  is formed between semiconductor substrate  10  and silicon layer  14 . In this embodiment, semiconductor substrate  10  is formed from silicon; however, semiconductor substrate  10  can be formed from different semiconductor materials in other embodiments (e.g., Gallium Arsenide). Further, in this embodiment, insulating layer  12  is formed from a silicon oxide (e.g., SiO 2 ), although in other embodiments insulating layer  12  can be formed from other non-conductive materials. 
     In one embodiment, semiconductor substrate  10 , insulator layer  12  and silicon layer  14  are formed using known silicon on insulator (SOI) wafer fabrication processes. For example, the buried oxide layer (i.e., insulating layer  12 ) can be formed using known oxygen implantation processes. Insulating layer  12 , in this embodiment, has a thickness of about 1 μm, but can range from about 0.35 μm to 2 μm in other embodiments. Further, in this embodiment, silicon layer  14  is about 2.5 μm, but can range from about 1 μm to 10 μm in other embodiments. 
     Protective layer  16  is formed on silicon layer  14 . In this embodiment, protective layer  16  is a silicon oxide formed using a suitable known process. For example, protective layer  16  can be formed by thermal oxidation of silicon layer  14 , or using a low temperature oxide (LTO) deposition process. In one embodiment, protective layer  16  is formed from oxide with a thickness of about 5 μm. In other embodiments, the protective layer can have a different thickness. A thickness greater than 1 μm helps prevent lateral growth of an epitaxial silicon layer formed in a subsequent stage (described below) in fabricating the taper. 
     Although an oxide protective layer is described above, in other embodiments, protective layer  16  can be formed from other materials (e.g., a silicon nitride material).  FIG. 1A  illustrates a top view of the resulting structure (not to scale), with protective layer  16  being the only layer that is visible. However, the area occupied by the rib waveguide (i.e., silicon layer  14 ) under protective layer  16  is indicated with dashed lines in FIG.  1 A. 
       FIG. 2  illustrates a partial cross-section of the semiconductor workpiece (not to scale) during another stage in fabricating a taper, according to one embodiment of the present invention. In this stage, a photoresist layer  21  is formed on protective layer  16  and is patterned to define the taper using known photolithographic processes. In this embodiment, photoresist layer  21  is patterned so that it forms the inverse of the taper. 
       FIG. 2A  illustrates a top view of the resulting structure (not to scale). As shown, photoresist layer  21  is patterned so that a portion of protective layer  16  is left uncovered. As will be described below, this uncovered portion defines the “foot print” of the taper to be formed in a subsequent stage of the taper fabrication process. In this embodiment, the wide end of the taper footprint has the same width as the waveguide formed by silicon layer  14 , although in other embodiments, the width may be different. Further, the shape of the tapers footprint may be different in other embodiments (e.g., triangular rather than pentagonal as in FIG.  2 A). The termination end of the taper footprint forms a relatively sharp angle (e.g., a few degrees), although the termination end may be truncated in other embodiments. 
       FIG. 3  illustrates a partial cross-section of the semiconductor workpiece (not to scale) during another stage in fabricating a taper, according to one embodiment of the present invention. In this stage, the uncovered portion of protective layer  16  ( FIG. 2 ) is etched so that silicon layer  14  is exposed, with the portion of protective layer  16  ( FIG. 2 ) under photoresist layer  21  ( FIG. 2 ) remaining intact. As shown in  FIG. 3 , the remaining portion of the protective layer is indicated as protective layer  16 A. 
     In one embodiment, a suitable known anisotropic etching process (e.g., a dry etching process such as reactive ion etching) is used to etch the portion of protective layer  16  ( FIG. 2 ) left uncovered by photoresist layer  21  (FIG.  2 ). In other embodiments, different etching processes can be used. Photoresist layer  21 . ( FIG. 2 ) is then stripped or removed using standard photolithographic processes. A partial cross section of the resulting structure is represented in FIG.  3 . As shown in  FIG. 3 , the termination end of the taper mask formed by protective layer  16 A is a point. In other embodiments, the termination need not be a point (e.g., the termination end may appear as in  FIG. 3  but with the point truncated). 
       FIG. 3A  illustrates a top view of the resulting structure (not to scale). As shown, protective layer  16 A is exposed after photoresist layer  21  ( FIG. 2 ) is removed. In addition, a portion of silicon layer  14  is exposed after the protective layer is etched. 
       FIG. 4  illustrates a partial cross-section of the semiconductor workpiece (not to scale) during another stage in fabricating a taper, according to one embodiment of the present invention. In this stage, a silicon layer  41  is formed on the exposed portion of silicon layer  14 . In one embodiment, a suitable known selective silicon epitaxy process in which silicon is “grown” on the exposed portion of silicon layer  14  while not growing on protective layer  16 A. In one embodiment, silicon layer  41  has a thickness of about 4 μm; however, in other embodiments silicon layer  41  can have a thickness of about 2 μm to about 8 μm. The optimal thickness of silicon layer  41  can depend at least in part on the width or diameter of the larger waveguide (e.g., optical fiber) to be coupled to the taper. The growth of silicon layer  41  is constrained by the side walls of protective layer  16 A so that silicon layer  41  is formed in the desired taper shape. In one embodiment, the selective silicon epitaxy process is terminated when the thickness of silicon layer  41  reaches the thickness of protective layer  16 A. In other embodiments, the growth of silicon layer  41  can be terminated before its thickness reaches that of protective layer  16 A. 
     This stage of the taper fabrication process represents a significant improvement over conventional processes that etch silicon to form the taper. For example, as previously described, etching the silicon undesirably results in erosion or “erosion-like” effects at the narrow or point end of the taper, increasing loss. In addition, the upper surface of the silicon waveguide may be undesirably “roughened” by the etching process (e.g., feature sensitivity), further increasing loss. 
     In contrast, by depositing silicon to form the taper in accordance with embodiments of the present invention, the point or narrow end of the taper is not eroded. Rather, the narrow end is essentially smooth and sharp, which tends to enhance performance of the resulting taper. In addition, the waveguide is not etched after protective layer  16  ( FIG. 2 ) is formed. Thus, the resulting upper surface of the waveguide (i.e. silicon layer  14 ) is significantly smoother than an etched surface. Consequently, the waveguide formed by silicon layer  14  will generally have less loss than one that is etched to form the taper. 
     In addition, selective silicon epitaxy processes can be sensitive to the surface topology of the growing surface (e.g., micro-loading). In one embodiment, this topology sensitivity is taken advantage of to form silicon layer  41  with a sloped upper surface. That is, the selective silicon epitaxy process will tend to grow silicon at a slower rate near the narrow end of the taper because at that end, the sidewalls of protective layer  16 A start getting closer and closer until they meet, changing the micro-loading in that area. As a result, the upper surface of silicon layer  41  will tend to slop downwards from the wide end of the taper to the narrow end of the taper as indicated by surface  41 A in FIG.  4 . This vertical slope of the taper can further increase the performance efficiency of the taper. 
     In other embodiments, a polysilicon layer can be deposited on protective layer  16 A and then planarized by chemical mechanical polishing (CMP) so that the upper surface of protective layer  16 A is exposed. However, this alternative embodiment can in some instances form the taper without the sloped upper surface that can be achieved using a selective silicon epitaxy process. 
       FIG. 4A  illustrates a top view of the resulting structure (not to scale). As shown, silicon layer  41  is visible, laterally surrounded by protective layer  16 A. In this embodiment, the termination end of the taper includes two surfaces that are angled so that the termination end is shaped like a wedge. As previously described, these surfaces of the wedge are not etched, which can advantageously increase the performance efficiency of the taper compared to conventional tapers that form the termination using etching processes. 
       FIG. 5  illustrates a perspective view of a section cut as indicated in  FIG. 4 , with protective layer  16 A omitted so that the taper (i.e., silicon layer  41 ) and the waveguide (i.e., silicon layer  14 ) can be more easily appreciated. In addition to protective layer  16 A, another protective layer (not shown) may be formed on silicon layer  41 . As shown in  FIG. 5 , the termination end (i.e., end  51 ) of the taper includes surfaces that are angled with respect to the longitudinal axis of the taper. In this embodiment, the longitudinal axis is along the line connecting the center of the termination end  51  of the taper to the center of the wide end  52  of the taper. 
       FIG. 6  illustrates a system  60  in which a waveguide taper according to embodiments of the present invention can be used. System  60  includes an optical signal source  61  connected to one end of an optical fiber  62 . The other end of optical fiber  62  is connected to a PLC  63  that includes a taper  64 . Taper  64  is fabricated according to one of the embodiments described above. For example, when the taper is implemented as shown in the embodiment of  FIG. 5 , wide end  51  would be used to connect PLC  63  to the end of optical fiber  62 . In one embodiment, PLC  63  is implemented in an integrated circuit. Other embodiments may have one or more other tapers (not shown) that are essentially identical in structure to taper  64 . 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable optical manner in one or more embodiments. 
     In addition, embodiments of the present description may be implemented not only within a semiconductor chip but also within machine-readable media or other electronic form. For example, the designs described above may be stored upon and/or embedded within machine readable media associated with a design tool used for designing semiconductor devices. Examples include a netlist formatted in the VHSIC Hardware Description Language (VHDL) language, Verilog language or SPICE language. Some netlist examples include: a behavioral level netlist, a register transfer level (RTL) netlist, a gate level netlist and a transistor level netlist. Machine-readable media also include media having layout information such as a GDS-II file. Furthermore, netlist files or other machine-readable media for semiconductor chip design may be used in a simulation environment to perform the methods of the teachings described above. 
     Thus, embodiments of this invention may be used as or to support a software program executed upon some form of processing core (such as the CPU of a computer) or otherwise implemented or realized upon or within a machine-readable medium. A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium can include such as a read only memory (ROM); a random access memory (RAM); a magnetic disk storage media; an optical storage media; and a flash memory device, etc. In addition, a machine-readable medium can include propagated signals such as electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.). 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.