Abstract:
An apparatus for self-aligning an optical fiber to an optical waveguide. The apparatus includes an optical waveguide chip including: one or more optical waveguides formed on a first substrate, each optical waveguide having a protruding portion; and one or more alignment rails formed on the first substrate, each alignment rail spaced apart from each optical waveguide by a predetermined distance; and an alignment jig including: one or more grooves formed in a second substrate, each groove adapted to receive one protruding portion and each groove supporting one optical fiber in alignment with one optical waveguide; and one or more alignment grooves formed on the second substrate, each alignment groove spaced apart from the grooves by the predetermined distance and adapted to mate with the alignment rails.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of coupling optical fibers to optical waveguides; more specifically, it relates to an apparatus and method for manufacturing self-aligned optical fiber to optical waveguide assembles.  
         BACKGROUND OF THE INVENTION  
         [0002]    There are many applications in optical communications and in optoelectronics where an optical fiber must be connected to an optical waveguide. Connection of an optical fiber to an optical waveguide requires alignment of a face of the optical fiber to the face of an optical waveguide and then bonding of the two faces together. Alignment tolerances of the optical fiber to the optical waveguide must be held to under 1 micron in five degrees of freedom (mutually perpendicular straight X, Y and Z axes as well as a rotational axes about the X and Y axes). The equipment to accomplish this five-fold alignment is expensive and requires a high degree of skill on the part of the person performing the alignment. Five-fold alignment is also time-consuming. Both of these add to the cost of the finished product. Further, with this technique it is extremely difficult or impossible to attach multiple optical fibers to an optical waveguide on very close pitches. An inexpensive method whereby the optical fiber aligns to the optical waveguide would greatly reduce the manufacturing costs of optical communication and optoelectronic devices.  
           [0003]    A need therefore exists for an inexpensive method of aligning in the aforementioned five degrees of freedom one or more optical fibers to an optical waveguide.  
         SUMMARY OF THE INVENTION  
         [0004]    A first aspect of the present invention is an apparatus for self-aligning an optical fiber to an optical waveguide comprising: an optical waveguide chip including: one or more optical waveguides formed on a first substrate, each optical waveguide having a protruding portion; and one or more alignment rails formed on the first substrate, each alignment rail spaced apart from each optical waveguide by a predetermined distance; and an alignment jig including: one or more grooves formed in a second substrate, each groove adapted to receive one protruding portion and each groove supporting one optical fiber in alignment with one optical waveguide; and one or more alignment grooves formed on the second substrate, each alignment groove spaced apart from the grooves by the predetermined distance and adapted to mate with the alignment rails.  
           [0005]    A second aspect of the present invention is an apparatus for self-aligning an optical fiber to an optical waveguide comprising: an optical waveguide chip including: one or more optical waveguides formed on a first substrate, each optical waveguide having a cladding layer extending over a top surface of the first substrate and a protruding portion, the protruding portion including a core portion and a cladding portion; and one or more alignment rails formed on top of the cladding layer, each alignment rail substantially co-planer with the core portion and spaced apart from each core portion by a predetermined distance; and an alignment jig including: one or more trenches in a thick layer on top of a second substrate and one or more grooves formed in the second substrate, each groove open to one trench; each groove adapted to receive one protruding portion and to support one optical fiber in alignment with one optical waveguide; and one or more alignment grooves formed in the thick layer, each alignment groove spaced apart from the grooves by the predetermined distance and adapted to mate with the alignment rails.  
           [0006]    A third aspect of the present invention is a method for making a self-aligned connection between an optical fiber and an optical waveguide, comprising: providing a first substrate; forming a first cladding layer on top of the first substrate; forming a core layer on top of the first cladding layer; etching the core layer to form a waveguide core and one or more alignment rails, each alignment rail spaced apart from the waveguide core by a predetermined distance; forming a second cladding on a top surface and on sidewalls of the waveguide core, the waveguide core and second cladding forming a protruding portion of the waveguide, the first cladding layer, the waveguide core and the second cladding forming the optical waveguide; providing a second substrate; forming a mask layer on top of the second substrate; simultaneously etching one or more trenches and one or more alignment grooves in the mask layer, each alignment groove spaced apart from the trench by the predetermined distance and adapted to mate with the alignment rails; etching a groove in the second substrate in each trench, each groove and trench adapted to receive one protruding portion and to support one optical fiber such that a core of the optic fiber aligns with the waveguide core; placing the first substrate onto the second substrate such that the alignment rails engage with the alignment grooves and the protruding portion is received in the groove and trench and placing optical fibers in each trench. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0007]    The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0008]    [0008]FIGS. 1 through 8 are partial cross-sectional views illustrating fabrication of an optical waveguide chip according to the present invention;  
         [0009]    [0009]FIG. 9 is a cross-sectional view of an optical waveguide chip according to the present invention;  
         [0010]    [0010]FIG. 10 is a three-dimensional isometric view of the optical waveguide chip of FIG. 9, according to the present invention  
         [0011]    [0011]FIGS. 11 through 17A are partial cross-sectional views illustrating fabrication of an alignment jig for aligning the optical waveguide chip of FIG. 9 to an optical fiber according to the present invention;  
         [0012]    [0012]FIG. 18 is a three-dimensional isometric view of an alignment jig of the present invention;  
         [0013]    [0013]FIG. 19 is a front view illustrating use of the alignment jig of FIG. 18 to align the optical waveguide chip of FIG. 10 to an optical fiber according to the present invention;  
         [0014]    [0014]FIG. 20 is a partial cross-sectional view through  20 - 20  of FIG. 19 illustrating use of the alignment jig of FIG. 18 to align the optical waveguide chip of FIG. 10 to the optical fiber of FIG. 19 according to the present invention;  
         [0015]    [0015]FIG. 21 is a front view illustrating use of an alignment jig to align an optical waveguide chip to a pair of optical fibers according to the present invention; and  
         [0016]    [0016]FIG. 22 is a partial cross-sectional view an alternative configuration of the optical waveguide chip and illustrating use of the alignment jig to align the alternative optical waveguide chip to an optical fiber according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    The processes and fabrication methods related and referenced herein are those used in the manufacture of semiconductor chips unless otherwise noted. The term optical waveguide is intended to include not only discrete optical waveguides but also optical devices having an optical or electro-optical device portion and an optical waveguide portion where both the optical waveguide portion and device portion are fabricated in or on the same substrate and are interconnected. The self-aligned optical waveguide to optical fiber connection system of the present invention requires fabrication of an integrally formed optical waveguide chip and fabrication of an alignment jig used to align the optical waveguide chip to an optical fiber. The fabrication of the optical waveguide chip is discussed first.  
         [0018]    [0018]FIGS. 1 through 8 are partial cross-sectional views illustrating fabrication of an optical waveguide chip according to the present invention. In FIG. 1, a first cladding layer  100  is formed on a top surface  105  of a substrate  110 . Substrate  110  may be silicon, sapphire or quartz. A core layer  115  is formed on a top surface  120  of first cladding layer  100 . In one example first cladding layer  100  is formed, by well-known oxidation or deposition methods, from thermal oxide, high-density plasma (HDP) oxide or tetraethoxysilane (TEOS) oxide boro-silicate glass (BSG) or phosphorus-boro-silicate (BPSG) glass, and is about 5 to 10 microns thick and has an index of refraction of about 1.44 to 1.54. In one example, core layer  115  is silicon-oxy-nitride, is 2 to 3 microns thick and has an index of refraction of about 1.47 to 1.55. However, the refractive index of core layer  115  is greater than the refractive index of first cladding layer  100 .  
         [0019]    In FIG. 2, photoresist images  125 A,  125 B and  125 C are formed, by well known photolithographic methods, on a top surface  130  of core layer  115 .  
         [0020]    In FIG. 3, a core  135  and rails  140  are formed from core layer  115  by removing portions of the core layer not protected by photoresist images  125 A,  125 B and  125 C (see FIG. 2) down to top surface  120  of first cladding layer  100  using any one of a number of well-known reactive ion etch (RIE) methods selective silicon oxy-nitride to silicon oxides. Photoresist images  125 A  125 B, and  125 C (see FIG. 2) are then removed by wet or dry stripping. First cladding layer  100 , core  135  and rails  140  extend perpenicularly into the plane of the drawing. This is more clearly illustrated in FIG. 10 and described below. Since rails  140  are formed at the same time as core  135 , the rails are self-aligned to the core.  
         [0021]    In FIG. 4, a photoresist image  145  is formed over core  135  and immediately adjacent portions  120 A of top surface  120  of first cladding layer  100  using well-known lift-off photolithographic techniques. Note that photoresist image  145  has angled sidewalls  150  such that the photoresist image is wider at a top surface  155  of the photoresist image than at portions  120 A of top surface  120  of first cladding layer  100 . A tapered photoresist sidewall is a well-know attribute of a lift-off photolithographic technique. A conformal protective layer  160  is formed on top surfaces  165  and sidewalls  170  of rails  140 , top surface  155  of photoresist image  145  and on exposed top surface(s)  120  of first cladding layer  100 . In one example, protective layer  160  is silicon nitride and is about 100 to 300 Å thick and is formed by any one of well-known sputtering, collimated sputtering or other non-conformal deposition processes.  
         [0022]    In FIG. 5, photoresist image  145  and that portion of protective layer  160  deposited on top surface  155  of the photoresist image (see FIG. 4) are removed using a solvent that will dissolve the photoresist image. Protective layer  160  covers all of top surface  120  of first cladding layer  120  except for exposed portions  120 B immediately adjacent to core  135 . The function of protective layer  160  is to protect rails  140  from subsequent processes.  
         [0023]    In FIG. 6 a second cladding layer  175  is formed on first cladding layer  100 , covering rails  140 , protective layer  160 , exposed portions  120 B of top surface  120  of first cladding layer  100  and core  135 . In one example second cladding layer  175  is formed, by well-known deposition methods, from HDP oxide or TEOS oxide, BSG or BPSG, is about 7 to 13 microns thick and has an index of refraction of about 1.44 to 1.54. However, the index of refraction of second cladding  175  is less than the refractive index of core  135 . In one, example, the refractive index of second cladding layer  175  is about the same as the refractive index of first cladding layer  100 . First cladding layer  100  and second cladding layer  175  may or may not be formed from the same material.  
         [0024]    In FIG. 7, a photoresist image  180  is formed on a top surface  185  of second cladding layer  175  and aligned over core  135  using well-known photolithographic techniques.  
         [0025]    In FIG. 8, an upper cladding  186  is formed from second cladding layer  175  by removing portions of the second cladding layer not protected by photoresist image  180  (see FIG. 7) down to protective layer  160  using any one of a number of well-known RIE methods selective silicon oxides to silicon nitride. Photoresist image  180  (see FIG. 7) is then removed by wet or dry stripping. Upper cladding  186  overlaps edge portions  190  of protective layer  160 . Protective layer  160  does not extend under upper cladding  185  all the way to core  135 . A lower cladding  195  is defined as an area of first cladding layer  100  aligned approximately to sides  192  of upper cladding  186 . Upper cladding  186  extends perpendicularly into the plane of the drawing. This is more clearly illustrated in FIG. 10 and described below.  
         [0026]    [0026]FIG. 9 is a cross-sectional view of optical waveguide chip  200  according to the present invention. In FIG. 9, optical waveguide chip  200  is cut from substrate  110  (see FIG. 8). Optical waveguide chip  200  includes an integrally formed optical waveguide portion  205 , alignment rails  230  and a supporting substrate  110 A. Optical waveguide portion  205  includes upper cladding  186  surrounding a top  210  and sidewalls  215  of core  135  and lower cladding  195  contacting a bottom  220  of the core and bottom surface(s)  225  of the upper cladding. Alignment rails  230  include rail  140  and portions of protective coating  160  covering exposed surfaces of the rail. Supporting substrate  110 A may contain semiconductor devices and circuits and/or electro-optical devices linked to optical waveguide  205 . Alignment rails  230  have a width “W1” and a height “H1.” In one example, W1 is about 5 to 10 microns and “H1” is about 2 to 3 microns. Core  135  has a width “W2” and a height “H1.” Top surfaces  232  of alignment rails  230  are lower than a top surface  233  of optical waveguide  205  by a distance “H3.” Optical waveguide  205  has a width “W3” and a height “H2.” In one example, W2 is about 5 to 10 microns, W3 is about 15 to 30 microns, “H2” is about 12 to 23 microns and “H3” is about 5 to 10 microns.  
         [0027]    [0027]FIG. 10 is a three-dimensional isometric view of optical waveguide chip  200  of FIG. 9, according to the present invention. In FIG. 10, at least a front surface  235  of optical waveguide chip  200  is polished optically flat and perpendicular to a longitudinal axis  240  of the optical waveguide chip. Front surface  235  includes ends  237  of alignment rails  230  and an end  238  of waveguide  205 . Alignment rails  230  and optical waveguide  205  extend along longitudinal axis  240 . Alignment rails  230  extend parallel to and are co-planer with core  135 . Centers of alignment rails  230  are spaced a distance “S1” from the center of optical waveguide  205 .  
         [0028]    While upper cladding  185  and core  135  are illustrated as extending the length of substrate  110 A, the upper cladding and core may terminate prior to reaching a back surface  242  of optical waveguide chip  200 .  
         [0029]    Fabrication of the alignment jig is now illustrated and described.  
         [0030]    [0030]FIGS. 11 through 17 are partial cross-sectional views illustrating fabrication of an alignment jig for aligning optical waveguide chip  200  to an optical fiber according to the present invention. In FIG. 11, a first hard mask layer  245  is formed on a top surface  250  of a (100) silicon substrate  255  having a &lt;100&gt; crystal orientation relative to the top surface. In one example first hard mask layer  245  is formed, by well-known oxidation or deposition methods, from thermal oxide, HDP oxide or TEOS oxide and is about 5 to 10 microns thick. The thickness of first hard mask layer  245  is slightly thicker the than the thickness of core layer  115  (see FIG. 2.)  
         [0031]    In FIG. 12, photoresist images  260  are formed, by well-known photolithographic methods, on a top surface  265  of first hard mask layer  245 .  
         [0032]    In FIG. 13, first trenches  270  and second trench  275  are formed in first hard mask layer  245  by removing portions of the first hard mask layer not protected by photoresist images  260  (see FIG. 12) down to top surface  250  of silicon substrate  255  using any one of a number of well-known RIE methods selective silicon oxides to silicon. Photoresist images  260  (see FIG. 12) are then removed by wet or dry stripping.  
         [0033]    In FIG. 14, a second hard mask layer  280  of CVD oxide or TEOS oxide about 300 to 600 Å thick is conformally deposited on first hard mask layer  245 . Alternatively, second hard mask layer  280  may be formed by a thermal oxidation of exposed silicon at the bottom of first trenches  270  and second trench  275 .  
         [0034]    In FIG. 15, photoresist images  290  are formed, by well-known photolithographic methods, on a top surface  285  of second hard mask layer  280  immediately adjacent to first trenches  270  and over the first trenches, but not over first trench  275 .  
         [0035]    In FIG. 16, portions of the second hard mask layer  280  not protected by photoresist images  290  (see FIG. 15) are removed in second trench  275  down to top surface  250  of substrate  255  by wet-etching in dilute or buffered HF. Photoresist images  290  (see FIG. 13) are then removed by wet or dry stripping.  
         [0036]    In FIG. 17, a V-shaped groove  295  is formed in silicon substrate  255  exposed in second trench  275  by etching in an aqueous or alcoholic solution of a strong base such as KOH, NaOH, tetramethylammonium hydroxide (TMAH) or ethylene diamine pyrocatechol (EDP.) Formation of V-shaped grooves in (100) silicon is well known. The V-shape is formed because the etch rate in the &lt;111&gt; crystallographic plane is faster than in any of the other planes. The depth “D1” (measured from top surface  250  of silicon substrate  255 ) of V-shaped groove  295  is primarily a function of width “W4” of second trench  275  and secondarily of etch time. “W4” must be at least equal to “W3” (see FIG. 9) plus an amount Δ1. The value of “W4” and etch time must also be adjusted such that a distance “D2” measured from an upper edge  300  of second trench  275  to a point  305  on sidewalls  310  of V-shaped groove  295 , directly below the upper edge is equal to “H3” plus an amount Δ2 (see FIG. 9). Sidewalls  310  meet along an edge  312  centered under second trench  275 . Alignment grooves  315  include first trenches  270  and portions of second hard mask layer  280  covering exposed surfaces of the trenches. Alignment grooves  315  have a width “W5” and a height “H4”) “W5” is equal to “W1” plus an amount Δ3 and “H4” is equal to “H1” plus an amount Δ4 (see FIG. 9). In one example, Δ1, Δ2, Δ3 and Δ4 are about 250 to 1000 Å and may or may not be equal to one another.  
         [0037]    [0037]FIG. 17A illustrates an alternative shape for V-shaped groove  295  of FIG. 17. In FIG. 17A, sidewalls  310  of V-shaped groove  295 A do not meet, but instead a flat bottom  317  is formed. Flat bottom  317  is formed simply by etching silicon substrate  255  for less time then that required for forming a “V.” 
         [0038]    [0038]FIG. 18 is a three-dimensional isometric view of alignment jig  320  of the present invention. In FIG. 18, alignment jig  320  is cut from substrate  255 . At least a front surface  325  of alignment jig  320  is polished optically flat and perpendicular to a longitudinal axis  330  of the alignment jig. Alignment grooves  315  and V-shaped groove  295  extend along longitudinal axis  330 . V-shaped groove  295  extends to a rear surface  335  of alignment jig  320 . Alignment grooves  315  terminate, prior to reaching rear surface  335 , in stops  340  of the alignment grooves. Alignment grooves  315  extend parallel to V-shaped groove  295 . The centers of alignment grooves  295  are spaced distance “S1” from the center of V-shaped groove  295 .  
         [0039]    [0039]FIG. 19 is a front view illustrating use of alignment jig  320  of FIG. 18 to align optical waveguide chip  200  of FIG. 10 to an optical fiber  345  according to the present invention. This view is from front surface  235  of waveguide chip  200  (see FIG. 10) and front surface  325  of alignment jig  320  (see FIG. 18). In FIG. 19, optical waveguide chip  200  is engaged into alignment jig  320 . Alignment rails  230  on optical waveguide chip  200  slidably engage alignment grooves  315  in alignment jig  320 . A portion of optical waveguide  205  is suspended in V-shaped groove  295 . Optical fiber  345  is slidably engaged in V-shaped groove  295 . Optical fiber  345  includes an optical fiber core  350  surrounded by an optical fiber cladding  355 . An outer surface  360  of optical fiber  345  contacts sidewalls  310  of V-shaped groove  295 . Core  135  of optical waveguide  205  is co-axially aligned to optical fiber core  350 . Alignment along “X” axis  365  is provided by alignment guides  230  engaging alignment grooves  315 . Alignment along “Y” axis  370  is provided by the depth of V-shaped groove  295 . Since alignment rails  230  are self aligned to optical waveguide  205  and the spacing of the alignment rails and optical waveguide can be replicated, to an extremely high degree of accuracy and precision by modern lithographic technology, in the spacing of alignment grooves  315  and V-shaped groove  295 , optical fiber  345  is essentially self aligned to the optical waveguide in the “X” and “Y” axes. In the case of a flat-bottomed V-shaped groove (see FIG. 17A) the location of flat bottom  317  relative to optical fiber  345  is indicated by a dashed line.  
         [0040]    [0040]FIG. 20 is a partial cross-sectional view through  20 - 20  of FIG. 19 illustrating use of alignment jig  320  of FIG. 18 to align optical waveguide chip  200  of FIG. 10 to optical fiber  345  according to the present invention. In FIG. 20, optical waveguide chip  200  is positioned in alignment jig  320  by sliding the waveguide chip along “Z” axis  375  until alignment rails  230  contact stops  340  of alignment grooves  315  (see FIGS. 10, 18 and  19 ). Optical fiber  345  is positioned in V-shaped groove  295  and slid toward front surface  238  of optical waveguide  205  until a front surface  380  of the optical fiber is a distance “D3” from front surface  238  of the optical waveguide. (Front surface  380  of optical fiber  345  has been polished perpendicular to longitudinal axis  385  of the optical fiber by any one of several processes known to those skilled in the art.) In one example, “D3” is about 0 to 2 microns. An optical epoxy  390  is used to bond front surface  380  of optical fiber  345  to front surface  238  of optical waveguide  205 .  
         [0041]    [0041]FIG. 21 is a front view illustrating use of alignment jig  320 A to align optical waveguide chip  200 A to a pair of optical fibers according to the present invention. In FIG. 21, optical waveguide chip  200 A includes a first integrally formed optical waveguide portion  205 A, a second integrally formed optical waveguide portion  205 B and a pair of alignment rails  230 A. Alignment jig  320 A includes a first “V’ groove  295 A, a second V-shaped groove  295 B, and a pair of alignment rails  230 A. Alignment rails  230 A of optical waveguide  200 A are engaged in alignment grooves  315 A of alignment jig  320 A. A first optical fiber  345 A is positioned in first V-shaped groove  295 A and a second optical fiber  345 B is positioned in second V-shaped groove  295 B. First optical fiber  345 A is self-aligned to a first integrally formed optical waveguide portion  205 A of optical waveguide chip  200 A. Second optical fiber  345 B is self-aligned to a second integrally formed optical waveguide portion  205 B of optical waveguide chip  200 A. While two optic fibers and two optical waveguide portions are illustrated in FIG. 21, more than two optical fibers may be aligned and connected to more than two optical waveguide portions.  
         [0042]    [0042]FIG. 22 is a partial cross-sectional view an alternative configuration of the optical waveguide chip and illustrating use of the alignment jig to align the alternative optical waveguide chip to an optical fiber according to the present invention. In FIG. 22, optical waveguide chip  200  is positioned in alignment jig  320  by sliding the waveguide chip along “Z” axis  375  until alignment rails  230  contact stops  340  of alignment grooves  315  (see FIGS. 10, 18 and  19 ). Optical fiber  345  is positioned in V-shaped groove  295  and slid toward front surface  238  of optical waveguide  205  until a front surface  380  of the optical fiber is a distance “D3” from front surface  238  of the optical waveguide. Front surface  238  of optical waveguide  205  has been polished to a predetermined angle to longitudinal axis  140  by any one of several processes known to those skilled in the art. Front surface  380  of optical fiber  345  has been polished at the same predetermined angle to longitudinal axis  385  of the optical fiber by any one of several processes known to those skilled in the art In one example, “D3” is about 0 to 2 microns. Optical epoxy  390  is used to bond front surface  380  of optical fiber  345  to front surface  238  of optical waveguide  205 .  
         [0043]    The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.