Abstract:
An optical connector system is disclosed which utilizes a pair of substrates having optical elements associated therewith. Fiducial features in conjunction with alignment members are further utilized to provide for a greater alignment of the optical elements. Methods for forming the optical connector system are also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Application Serial No. 60/255,866, filed Dec. 14, 2000 the contents of which are herein incorporated by reference. 
     
    
     
       BACKGROUND  
         [0002]    1. Technical Field  
           [0003]    The present disclosure relates to a stacked chip structure. More particularly, this disclosure is directed to a structure for stacking chips in an accurate alignment without performing a backside alignment procedure on the bottom surface of the top chip.  
           [0004]    2. Background of Related Art  
           [0005]    In general, assemblies utilizing silicon include those which employ a frontside/frontside alignment. For example, as shown in FIG. 1, an optical assembly can be formed by employing two silicon substrates  10  and  30 . Top substrate  10  has an optical element  12  and fiducials  14  and  16  formed in the bottom surface  11  of substrate  12  for receiving alignment spheres  18  and  20  with the bottom surface being patterned. Bottom substrate  30  has an optical element  32  and fiducials  34  and  36  formed in the top surface  31  of substrate  30  for receiving alignment spheres  18  and  20  with the top surface being patterned. The optical elements  12  and  32  associated with substrates  10  and  30  can be aligned by way of the alignment spheres  18  and  20  received in the respective fiducials of substrates  10  and  30 . However, it is sometimes necessary to align an optical device on the upper surface of the top substrate with an optical device on the upper surface of the bottom substrate as, for example, when signals are transmitted through the substrate. Such an alignment can pose difficulties.  
           [0006]    For example, as shown in FIG. 2, when a top substrate  100  has an optical element  120  formed in the top surface  130  of substrate  100  with the top surface being patterned instead of in the bottom surface, then a backside alignment procedure must be performed on the bottom surface  110  of substrate  100  to form fiducials  140  and  150  for receiving alignment spheres  160  and  170  to align-top substrate  100  with bottom substrate  30 . Problems associated with this type of alignment procedure is that inaccurate alignment will result. Typically, this type of alignment procedure will be accurate to within only 5 microns. However, for micromechanical or microoptical devices, alignment of the substrates should be within about 1 micron or lower.  
           [0007]    It would be desirable to provide a more accurate alignment of two substrates for forming an optical connector system without performing a backside alignment procedure on the bottom surface of the top substrate.  
         SUMMARY  
         [0008]    A stacked assembly is provided herein, the stacked assembly comprising first and second substrates, each having an upper surface and a lower surface. The first and second substrates are arranged in superposed, adjacent relation such that the lower surface of the first substrate is adjacent the upper surface of the second substrate. A first alignment rod is operatively associated with the upper surface of the first substrate. The stacked assembly includes means operatively associated with the upper surface of the second substrate for engaging the alignment rod such that the alignment rod and engaging means cooperate to arrange the first and second substrates at a predetermined orientation.  
           [0009]    The stacked assembly provided herein advantageously can be formed by aligning the first substrate with the second substrate without performing a backside alignment procedure. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    Various embodiments of the optical connector system of the present disclosure are described below with reference to the drawings wherein:  
         [0011]    [0011]FIG. 1 is a schematic cross-sectional representation of a prior art alignment for a frontside-frontside alignment optical connector system;  
         [0012]    [0012]FIG. 2 is a schematic cross-sectional representation of a prior art alignment for a frontside-backside alignment optical connector system;  
         [0013]    [0013]FIG. 3 is a perspective view of one embodiment of the optical connector system in accordance with the present invention;  
         [0014]    [0014]FIG. 4 is a top view of the optical connector system of FIG. 3;  
         [0015]    [0015]FIG. 5 is a schematic cross-sectional view of a portion of the optical connector system of FIG. 3  
         [0016]    [0016]FIG. 6 is a schematic cross-sectional view of an alternative elongated alignment member for forming the optical connector system of the present invention;  
         [0017]    FIGS.  7 - 11  are top views of alternative embodiments of optical connector systems in accordance with the present invention;  
         [0018]    [0018]FIG. 12 is an elevational view of an embodiment employing an alignment post;  
         [0019]    [0019]FIG. 13 is a top view of an embodiment of the invention wherein the upper substrate has openings through which the alignment rods can engage alignment spheres; and,  
         [0020]    [0020]FIG. 14 is a sectional view showing an embodiment wherein the alignment rods have been removed after assembly. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    As used herein such terms as “upper” and “lower” or “top” and “bottom” are used relative to each other and not to any external fixed frame of reference.  
         [0022]    The present stacked assembly is useful for aligning a wide range of different signal communication elements. For example, the connector system described herein can be used to align fiber arrays or a fiber array and a micro lens array. The present invention can also be used to align, for example, micromachined substrates for micromechanical or microoptical devices.  
         [0023]    Turning now to the drawings, FIG. 3 illustrates, in perspective view, one embodiment in accordance with the present invention. FIG. 4 is a plan view of the embodiment shown in FIG. 3. Stacked assembly system  200  includes at least substrates  220  and  300  with substrate  300  having generally larger dimensions (e.g., length and width) than substrate  220 . Substrates  220  and  300  can be any suitable material capable of being processed to form the requisite alignment fiducials therein, as discussed below. Suitable materials include, but are not limited to, silicon, gallium arsenide (GaAs), metals, polymeric materials such as, for example, a high-performance engineering plastic, and the like. Silicon is a preferred substrate material.  
         [0024]    First substrate  220  is preferably fabricated from single crystal silicon and possesses a top surface  221  and bottom surface  222 . Top surface  221  will generally include at least one elongated alignment fiducial  230  for receiving elongated alignment rod  235 . The alignment fiducial  230  can be any feature (e.g., a ridge or groove, or a series of projections, etc.) which serves to orient the alignment rod  235  in a predetermined direction when engaged therewith. As shown herein, elongated alignment fiducial  230  is formed as grooves which extend along the top surface  221  of substrate  220  and terminates at the peripheral edge thereof. The groove  230  can be fabricated by employing well known etching techniques, e.g., anisotropic etching process utilizing a conventional anisotropic etchant known to those with skill in the art such as potassium hydroxide (KOH) or ethylene diamine pyrocatechol (EDP). In this manner, the depth and width of the V-groove is controlled with great precision. This enables a highly accurate passive alignment of substrate  220  with a second substrate  300  when elongated member  235  is received in gap  340  between alignment members  330  and  335  associated with substrate  300  as discussed below. Suitable alignment rods  235  can be formed from any conventional materials known in the art. Such materials include, but are not limited to, ceramics (e.g., silicon nitride, alumina, carbides, etc.), and metals (e.g., steel, titanium, etc., and the like). Alignment rod  235  is advantageously provided in the shape of cylindrical rods having circular cross sections to provide for more accurate alignment of substrate  220  with substrate  300 . Alternatively, alignment rods  235  can have a prismatic configuration with triangular, square, hexagonal, or other polygonal cross sections, as well as oval cross sections.  
         [0025]    The top surface  221  of substrate  220  can also include a signal communication element  225  mounted thereto for emitting, receiving, carrying, transmitting, or modifying a signal. Optionally, element  225  is an optical element such as, for example, a lens, filter, optical fiber, laser diode, photodetector, and the like.  
         [0026]    Silicon substrate  300  possesses top surface  305  and bottom surface  310 . Top surface  305  of substrate  300  includes means for engaging the alignment rod  235 . Such means for engaging the alignment rod can be any member (or plurality of members) having one or more surfaces which can be contacted by the alignment rod and which cooperate with the alignment rod to maintain the substrate  200  in a fixed orientation relative to substrate  300 . In the present embodiment the means for engaging the alignment rod include alignment members  330  and  335 . The substrate  300  includes at least fiducials  320  and  325  for receiving alignment members  330  and  335 , respectively. As discussed above, substrate  300  can be formed from the same or different materials as substrate  220 .  
         [0027]    Alignment members  330  and  335  have alignment surfaces  331  and  336 , respectively, which are contacted by the alignment rod  235 . If one imagines a vertical plane extending through the center of the alignment rod  235 , alignment surfaces  331  and  336  contact the alignment rod  235  on opposite sides of the plane so as to cradle the alignment rod  235 , holding the alignment rod in a predetermined orientation.  
         [0028]    Fiducials  320  and  325  are configured and dimensioned to receive and align corresponding alignment members  330  and  335 . The alignment members  330  and  335 , in conjunction with alignment rod  235 , provide alignment of the optical element of substrate  300  with the optical element of substrate  220 . Techniques and parameters for forming fiducials  320  and  325  are within the purview of one skilled in the art (e.g., by anisotropic, isotropic, or dry etching of silicon). Fiducials  320  and  325  are adjacent to one another such that alignment members  330  and  335  are also adjacent one another.  
         [0029]    Alignment members  330  and  335  are shown and described herein as spheres, although any other shape capable of performing the function described herein (e.g., cube, pyramid, rectangle, cylinder, etc.) is also contemplated as being within the scope of the invention. Alignment spheres  330  and  335  are highly precise balls fabricated from, for example, materials which include, but are not limited to, glass, ceramics, silicon nitride, alumina, carbides, metals (e.g., titanium, steel, aluminum, etc.), plastics, and the like. Alignment spheres  330  and  335  have a diameter optionally ranging from about 0.05 mm to about 2.0 mm and a diameter tolerance of optionally no more than about ±0.5 microns. Tolerance can vary depending on the material used to fabricate the alignment sphere. It will be recognized that the diameters of the alignment spheres can be outside the range given above without departing from the spirit or scope of the invention.  
         [0030]    An advantageous feature of the method described herein is the self-centering nature of the alignment. For example, if the alignment spheres have a larger or smaller diameter than expected, the position along the Z-axis (i.e., a vertical axis) of the contact points between the alignment rod and the alignment spheres can be affected. But the X and Y axis positions (the location of the contact points in a lateral horizontal plane) is relatively unaffected. Usually, the Z-axis position of the contact points is not as critical as the lateral position. It should be noted that alignment spheres are usually made in large batches, and the alignment spheres within any single batch usually exhibit a high degree of consistency in diameter.  
         [0031]    When substrate  220  and substrate  300  are assembled the bottom surface of substrate  220  is brought into facing relationship to the top surface of substrate  300  such that the alignment rod  235  is advantageously received in gap  340  between alignment spheres  330  and  335 , and is in contact with the alignment spheres. FIG. 5 is a side view of a portion of the stacked assembly showing the engagement of the alignment rod and the spherical members  330  and  335 .  
         [0032]    [0032]FIG. 6 shows an alternative embodiment  236  of an alignment rod having longitudinally extending flat surfaces  235   a  and  235   b  corresponding to and in contact with the side surfaces of the V-groove  230 , and further in contact with alignment spheres  330  and  335  in gap  340 .  
         [0033]    Once alignment rod  235  is received in gap  340 , alignment rod  235  can then be fixedly secured to alignment spheres  330  and  335  by employing any suitable attachment technique well known to one skilled in the art. These techniques such as soldering, brazing, use of bonding agents (e.g., epoxies or other monomeric or polymeric adhesives, sol gel glass, etc.), eutectic bonding, thermo-compression bonding, ultrasonic bonding, thermo-sonic bonding, or any other attachment technology known to those with ordinary skill in the art. These same technologies can be used to bond the alignment rod  235  to top substrate  220 , and the alignment spheres  335  and bottom substrate  300 . Likewise, when the substrates  220  and  300  are in a desired configuration, a bonding agent or other suitable bonding means can be used to securely fix the relative positions of the aligned substrates to prevent relative movement and to maintain the desired alignment.  
         [0034]    While FIGS. 3 and 4 illustrate alignment of substrates  220  and  300  utilizing four alignment rod  235  received in four sets of alignment spheres  330  and  335  arranged such that a stable mechanical attachment is achieved, it is to be understood that any suitable arrangement and number of elongated members  235  and spheres  330  and  335  may be utilized which results in a stable mounting of substrate  200 . The mounting can optionally be kinematic in design. For example, referring to FIG. 7, three alignment rods  235  can be received in three sets of alignment spheres  330  and  335  to align substrate  220  with substrate  300 . Other examples of alternative embodiments of the present invention are illustrated in FIGS.  8 - 11 .  
         [0035]    For example, FIG. 8 illustrates an optical connector system similar to that of FIG. 7 except that one or more set of fiducials ( 320 ,  325 ) are oriented along a diagonal direction relative to the substrate  300  for reception of alignment spheres  330  and  335 .  
         [0036]    [0036]FIG. 9 shows an embodiment wherein alignment rods  425  and  430  are mounted within fiducial grooves  420  in substrate  400 . However, alignment rod  425  has two opposite end portions  425   a  and  425   b , which extend beyond the periphery of substrate  400 . End portion  425   a  contacts alignment spheres  511  and  521 , which are mounted to bottom substrate  500  in fiducials  510  and  520 , respectively. End portion  425   b  contacts alignment spheres  531  and  541 , which are mounted to bottom substrate  500  in fiducials  530  and  540 , respectively. Alignment rod  430  is perpendicular to alignment rod  425  and has an end portion extending beyond the periphery of substrate  400 . The end portion of alignment rod  430  contacts alignment spheres  551  and  561 , which are mounted in fiducials  550  and  560 , respectively in bottom substrate  500 .  
         [0037]    [0037]FIG. 10 shows an embodiment similar to that shown in FIG. 9 except that alignment rod  430  is oriented parallel to alignment rod  425  and has two opposite end portions  430   a  and  430   b . End portion  430   a  contacts alignment spheres  551  and  561 , which are mounted to bottom substrate  500  in fiducials  550  and  560 , respectively. End portion  430   b  contacts alignment spheres  571  and  581 , which are mounted to bottom substrate  500  in fiducials  570  and  580 , respectively. Top substrate  400  can be movably adjusted in a linear direction parallel to the alignment rods  425  and  430 .  
         [0038]    [0038]FIG. 11 is a-top plan view illustrating an embodiment wherein top substrate  600  includes two spaced-apart alignment rods  611  and  621  which are parallel to each other, and an alignment rod  631  which is substantially perpendicular to the orientation of alignment rods  611  and  621 . Alignment rod  611  is mounted in fiducial groove  610  and has two opposite end portions  611   a  and  611   b , which extend beyond the peripheral edge of substrate  600 . Alignment rod  621  is mounted in fiducial groove  620  and has two opposite end portions  621   a  and  621   b , which extend beyond the peripheral edge of substrate  600 . Alignment rod  631  is mounted in fiducial groove  630  and has an end portion  631   a  extending beyond the peripheral edge of substrate  600 . End portions  611   a  and  621   a  contact opposite sides of a single alignment sphere  656 , and end portions  611   b  and  621   b  contact opposite sides of alignment sphere  651 . End portion  631   a  contacts two alignment spheres  652  and  653 . Alignment sphere  652  is mounted within fiducial  655  in substrate  650  and alignment sphere  653  is mounted within fiducial  654  in substrate  650 .  
         [0039]    Referring to FIG. 12, optical connector system  700  includes a first substrate  710  having an alignment groove  713  on an upper surface  711 . An alignment rod  730  is disposed in the alignment groove  713 . Second substrate  720  includes an alignment post  725  having an alignment fiducial  723  defined by alignment surfaces  723   a  and  723   b . The alignment rod  730  contacts the alignment surfaces  723   a  and  723   b , and is held in a desired orientation, thereby aligning first substrate  710  and second substrate  720 . Once aligned, substrates  710  and  720  can be fixedly secured in the desired orientation by a bonding agent or other suitable fixation means. Once the substrates are fixed in their position relative to each other, the alignment rod  730  can be removed, if desired, and reused to align another stacked substrate assembly. The alignment post  725  can be made in a silicon substrate by, for example, anisotropic etching with a suitable etchant (e.g., potassium hydroxide).  
         [0040]    Referring now to FIG. 13, an optical device package  800  includes a first substrate  810  and a second substrate  820 . First substrate  810  includes an upper surface having alignment grooves  811  and  812 , and openings  815 ,  816 , and  817  extending from the upper surface to the lower surface of substrate  810 . Second substrate  820  includes fiducials  821  and  822  with alignment spheres  841  and  842  respectively positioned therein, fiducials  823  and  824  with alignment spheres  843  and  844  respectively positioned therein, and fiducials  825  and  826  with alignment spheres  845  and  846  respectively positioned therein. The fiducials and alignment spheres are positioned so as to align with corresponding openings  815 ,  816  and  817 . Alignment rod  830  is disposed in groove  811 , and the opposite end portions of alignment rod  830  laterally extend through openings  815  and  816  to contact alignment spheres  841  and  842  at one end, and alignment spheres  842  and  844  at the other end. Alignment rod  831  is positioned in alignment groove  812 . One end portion of alignment rod  831  extends laterally through opening  817  to contact alignment spheres  845  and  846 . An optical signal communication element, i.e., lens  850 , is disposed on the upper surface of the first substrate  810 . As can be seen, in this embodiment the first and second substrates  810  and  820  can be of the same dimensions of length and width without one substrate extending beyond the periphery of the other substrate.  
         [0041]    Referring now to FIG. 14, an optical device package  900  includes a first substrate  910  having an upper surface with a lens  950 , and a second substrate  920  fixedly secured to the first substrate  910  in an aligned configuration. Alignment spheres  941  and  942  are mounted respectively in fiducials  921  and  922  in the second substrate. An optical fiber  960  extends through the second substrate  920 . An optical signal of appropriate wavelength (e.g., infrared) can be transmitted along an optical axis normal to the planes of substrates  910  and  920  from the optical fiber  960  through the silicon substrate  910  to lens  950 . As can be seen, alignment rods, once used to align the substrates  910  and  920 , can be omitted from the final optical device package  900  after the substrates  910  and  920  are fixed in the aligned relative positions by a suitable bonding agent or fixation means.  
         [0042]    Although the invention has been described in its preferred formed with a certain degree of particularity, many changes and variations are possible therein and will be apparent to those skilled in the art after reading the foregoing description. For example, while the connector system has been described herein with respect to optical connectors, the present system can be used in other applications such as, for example, for semiconductor connectors. It is therefore to be understood that the present invention may be presented otherwise than as specifically described herein without departing from the spirit and scope thereof.