Abstract:
A novel optoelectronic packaging submount arrangement which incorporates a 90° C. electrical conductor turn, and more specifically methods of producing optoelectronic packaging submount arrangement incorporating 90° C. electrical conductor turns.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the provision of a plurality of diverse and novel optoelectronic packaging submount arrangements each of which incorporates a 90° C. electrical conductor turn. Moreover, the invention is further directed to the provision of novel methods of producing diverse optoelectronic packaging submount arrangements each incorporating 90° C. electrical conductor turns. 
     In order to conserve space in optoelectronic packaging it is generally desirable, and even frequently necessary, to position the optical fibers delivering the signals in a plane which is nearly coincident with the plane of the electronic board on which the transmitter, receiver, or transceiver module is based. A packaging problem which is perennially encountered in industry is presented by the fact that light must impinge against the face of receiver chips, while it is emitted from the face of VCSEL (vertical-cavity surface-emitting laser) chips which are employed as light emitters, so that when these optoelectronic chips are mounted on the electronic board, the light paths associated with the chips extend perpendicular to the axes of the optical fibers. This problem may be solved by either mounting the chips in the plane of the board and bending the optical path between the chips and the fibers by 90° C., or alternatively, by mounting the chips perpendicular to the plane of the board and bending or turning the electrical conductor paths between the chips and the board by 90° C. 
     2. Discussion of the Prior Art 
     In essence, the solution to the above-mentioned problems has been successfully pursued in the technology. However, the optical bend presents a challenging optical design problem, and the component which is utilized for the implementing of the bending process can be expensive and difficult to fabricate, as is described in M. S. Cohen et al. “Packaging Aspects of the Jitney Parallel Optical Interconnect”, Proc. 48 th  ECTC, pp. 1206-1215, 1998. In contrast with optical bending, the bending of an electrical conductor through an angle of 90 ° C. is relatively easily accomplished. For instance, a common implementation of such an electrical conductor bend of optoelectronic packaging involves the use of a flexible circuit, as described in P. Rosenberg, et al. “The PONI-1 Parallel-Optical Link”, Proc. 49 th  ECTC, pp. 763-769, 1999. However, such a flexible circuit implementation is costly and often is difficult to manipulate within very confined spaces. These frequently encountered problems are exacerbated as the data rates of the optoelectronic packaging increase, and consequently, the electrical paths are required decrease to in their lengths. 
     In order to solve this problem it has heretofore been proposed that electrical bending be used, but that each optoelectronic chip be mounted on a submount which incorporates electrical traces which provide a built-in 90° C. turn. The basic concept of a submount containing electrical traces with a built-in 90° C. turn, of course, is not new in the technology inasmuch as such submounts are commercially available, and for example, can be readily produced by depositing traces around a corner on a ceramic piece using thick-film screening technology. 
     SUMMARY OF THE INVENTION 
     Although basic principles in the production of optoelectronic packaging submount arrangements which incorporate 90° C. electrical conductor turns are generally known and employed in the technology, the present invention improves upon various important aspects thereof. In particular, pursuant to the invention, a number of advantages are derived thereby, as follows: 
     (a) A large number of submounts can be concurrently fabricated in parallel on a silicon wafer through the use of photolithography. This, in essence, will considerably reduce manufacturing expenditures so as to render it highly cost-effective. 
     (b) The dimensions of the traces being formed can be controlled precisely by the employment of the photolithography, in view of which they can be designed for high data-rate operation. 
     (c) The silicon material which is used for the submount facilitates a good degree of heat dissipation from the optoelectronic chip. 
     (d) The silicon material which is used for the submount, if desired, enables the fabrication of active devices on the submount. 
     (e) Finally, it is also possible to etch features into the submount which will aid in the packaging. 
     Accordingly, it is an object of the present invention to provide an optoelectronic packaging submount incorporating a 90° C. electrical conductor turn. 
     Another object of the present invention is to provide a plurality of optoelectronic packaging submounts having 90° C. electrical conductor turns as described herein, which may be concurrently and in parallel manufactured on a silicon wafer. 
     Yet another object of the present invention is to provide novel methods of producing optoelectronic packaging submounts having 90° C. electrical conductor turns in a highly economical and cost-effective manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference may now be made to the accompanying drawings representative of the production of optoelectronic packaging submounts pursuant to the invention; in which: 
     FIG. 1 illustrates, in a schematic representation, the fabrication of submounts on a silicon wafer; 
     FIGS. 2 a  and  2   b  illustrate, respectively, side and front views of a completed submount; 
     FIGS. 3 a  and  3   b  illustrate, respectively, side and front views of the die and wire bonding of a VCSEL on a submount; (a photodetector chip may be mounted in a similar fashion); 
     FIGS. 4 a  and  4   b  illustrate, respectively, side and front views of the submount modified for the flip-chip mounting of a VCSEL (a photodetector chip may be mounted in a similar fashion); and 
     FIGS. 5 a  and  5   b  illustrate, respectively, side views representing the arrangement of FIGS. 4 a  and  4   b  together with the incorporation of a fiber to convey light to or from the optoelectronic chip. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring in specific detail to the drawings, and particularly to FIG. 1, the following steps are employed in a procedure of fabricating the optoelectronic submounts  10 : 
     1. Conductive traces  12  and associated electroplating pads  14  are delineated on all of the (undiced) chips  16  of a wafer using standard lithographic technology. 
     2. The lithography as set forth in step 1 also serves to delineate conductors  18  in the kerf region  20  between rows of chips  16 . These conductors  18  are electrically connected to traces  12  in the chips  16 . For example, as shown in FIG. 1, the traces  12  are connected in every other row of chips  16 . 
     3. A second lithographic operation is used to create a photoresist mask (not shown) covering all regions of the chips  16  except for small windows  22  opened over the traces proximate the chip edge  24 . Each window  22  starts about 1 to 20 μm from the chip edge  24  and extends about 50-100 μm further away from the chip edge, as shown in the dark regions in FIG.  1 . 
     4. The wafer is immersed in an electroplating bath, contacts are made to the traces  12  located in the kerfs and the window regions  22  are plated up to a thickness of about 50-100 μm. 
     5. The resist is removed and the wafer is diced into the chips  16 . During dicing, all of the conductors  18  in the kerf regions  20  are sawn away. 
     Alternatively, instead of making contact with the traces  12  which are being electroplated by the method set forth hereinabove, the conductors  18  in the kerf regions  20  can also be omitted. Instead of the foregoing, a very thin metal film may be blanket deposited on the bare wafer in the first processing step; in which this thin layer serves to provide electrical contact for the electroplating step. The other processing steps follow as outlined hereinabove, however the final step before dicing; in effect, after resist removal; would be implementing the etch-off of the blanket thin-film conductive layer. Even if some of the desired metallurgy were to be inadvertently removed by this etching step it would be of no consequence, since only a short etch is needed to remove the thin metal of the conductive layer. 
     Referring to FIGS. 2 a  and  2   b , there is shown a completed submount  30  in which, for purposes of illustration only, typical dimensions are given (in mm) which may be appropriate for VCSEL mounting. It is ascertainable that the traces  12  have been thickened in each electroplated region  22  to about 50 or 100 82 m. The optoelectronic chip  16  can now be diebonded (and wirebonded) on the submount  30  following the usual procedures known in the technology. 
     purpose of the thickened electroplated regions  22  on the traces  12  is to provide good anchoring regions for soldering to the wiring board. The traces on the board are to be provided with solder bumps. Heat is applied, the bumps are melted, and the submount  30  is lowered into the solder bumps so that the solder wets the electroplated regions before freezing. Care must be taken to choose a metallurgy which will permit electroplated regions to be robust, yet solderable. Such a metallurgy may be a thick layer of copper or nickel, followed by a layer of gold. 
     Alternatively, the thickened electroplated regions  22  on the traces  12  may represent solder bumps which are electroplated. In this case the solder bumps on the submount are soldered to the solder bumps on the board, i.e., the two juxtaposed solder bumps are fused together. 
     Furthermore, it may be noted that in production the typical accuracy for placement of a sawcut has been ±20 μm, although precision wafer saws have a placement accuracy of ±1 μm. Since sawing into the thick electroplated layer could damage the latter, the electroplated region  22  is designed to be displaced from the nominal chip edge  24  by 1-20 μm, depending on the precision of the saw. Since the solder bump on the board may have a height of as much as 100-300 μm, the electroplated region will nevertheless be immersed in the solder. 
     Although the solder will provide a secure electrical connection, the solder bond may not be adequate for mechanical integrity. For this reason an adhesive, such as an epoxy, may be applied to produce a robust fillet between the submount and the board, such as on the opposite side of the submount  30  from the traces  12 . 
     The optoelectronic chips  16  which are to be die bonded to the submount  30  are preferably attached with AuSn eutectic solder, which may be applied without use of solder flux, and which evidences a good stability. It is therefore necessary to identify a solder for the solder bump which melts at a lower temperature than AuSn (280° C.) so that the positions of the optoelectronic chips  16  are not disturbed during solder-bump reflow. Since a commonly used earlier employed solder, such as 97%/3% PbSn has a melting point of about 300° C., a lower temperature solder is needed. In this connection, recently developed Pb-free solders may be used for this application; for example, an SnAgCu solder which has a melting point of 217° C. Other solder options are: Sn/5%Sb (mp: 235 C.), Sn/Bi/Ag (92 Sn, 4-5% Bi, 3-4% Ag; mp:211° C.). The added advantage of these solder systems is that they enable these packages to be Pb-free which, in industry, is increasingly a requirement for electronic and optoelectronic packages. 
     Alternatively, it is of course possible to employ electrically conductive materials other than solder in order to make the electrical connection between the submount and the board. Such materials could include conductive epoxies of various types. 
     As illustrated in FIGS. 3 a  and  3   b , there is represented an example as to the manner in which an optoelectronic chip  16  could be attached to the submount  30  where an active-side up VCSEL chip  16  is shown diebonded to the substrate  30  with a wirebond to an appropriate trace  12 . Here, it is assumed that the bottom face  32  of the VCSEL  16  (contacting the submount) represents one of its electrical terminals, whereas the other VCSEL terminal lies in its top (active) face  34 , and is accessible by a wire bond. The light is emitted from the top face  34  in a direction away from the submount. A receiver chip can be attached to a submount in similar fashion, with its active face away from the substrate and the appropriate electrical connections made by wire bonds. 
     Alternatively, however, either the VCSEL  16  or the receiver chip can be flip-chip bonded to the submount  30  so that the electrical connections would be made by direct solder connections to the traces. As is common for optoelectronic chips, AuSn solder could be used. A flip-chip mounting may be advantageous because the inductance associated with the wire bond is eliminated, thereby facilitating the achievement of higher data rates. However, since the silicon substrate is opaque at the shorter wavelengths often used in datacom applications (850 nm) it may be necessary to modify the submount to permit a free lightpath, with such a modification being illustrated in FIGS. 4 a  and  4   b.    
     In that case, holes  36  are etched in the silicon substrate  30 , and the optoelectonic chips  16  are mounted so as to straddle these holes. A standard anisotropic etching process may be used for this operation, as described in E. Bassous, “Fabrication of novel three-dimensional microstructures by the anisotropic etching of (100) and (110) silicon”, IEEE. Trans Electron Devices, vol, ED25, pp 1178-1185; 1978. 
     However, care must be exercised to avoid damaging the traces during the hole-etching process, since a caustic solution is commonly used for etching. For this purpose, the following procedure is suggested, although alternative procedures may also be possible: 
     1. Deposit an inert base film (silicon nitride) on the front side of the wafer (the side view of FIG. 4 a , i.e., the side to be used for delineating the traces). 
     2. Deposit the silicon-etch mask film (silicon nitride) on the back side of the wafer (view of FIG. 4 b ). Lithographically pattern that film so as to create the appropriate openings in the mask film for the silicon etching. 
     3. Etch the required holes in the silicon wafer. It is noted that the base film will remain stretched over the holes on the front side of the wafer. 
     4. Photolithograpically pattern the traces on the front side of the wafer. Because of the presence of the base film, photoresists may be applied successfully for lithography; this would not be the case in the absence of the base film because holes in the silicon would thwart the photoresist spinning operation. 
     5. Flip-chip bond the opoelectronic chips. Because of its transparency, the thin base film need not be removed under the chips to provide a free optical path. 
     It is noted that frontside-to-backside mask alignment is required for the above procedure. It is generally difficult to provide such alignment with high accuracy. However, high accuracy is not required in the present application because the etched hole need only be positioned accurately enough to ensure proper die bonding of the optoelectronic chip along with a free optical path. 
     Consideration must be given to the necessity of having to align each optoelectronic chip with its associated optical path, i.e., fiber. This alignment may be accomplished actively, such as by activating the VCSEL or the receiver chip, providing relative motion between the optoelectronic chip and the optical path until the electrical signal is optimized, then fixing the components in place in order to maintain their positions. Whereas active alignment can be carried out using the concepts outlined above, passive alignment may be a more attractive alignment procedure because of cost savings. In passive alignment the optoelectronic chip is not activated, but alignment is carried out with the aid of either (a) an mechanical alignment feature or (b) fiducial marks used as reference features. Two methods of passive alignment utilizing the optoelectronic submount are set forth hereinbelow, each of which entails butt-coupling a fiber to the optoelectronic chip (no lens involved). 
     (a) Passive Alignment Using Mechanical Features 
     Methods for fabricating fiber guides directly on optoelectronic chips have been described in copending U.S. patent application Ser. No. 09/479,109, filed Jan. 7, 2001, the disclosure of which is incorporated herein by reference. If such guides were fabricated on the chips, they would be accessible for fiber insertion either for active-side up, as in FIGS. 3 a  and  3   b  or active-side down, as in FIGS. 4 a  and  4   b . In the latter instance, the fiber would be inserted into the fiber guide through the hole which is etched in the silicon. 
     (b) Passive Alignment Using Precision Die Placement Tool 
     Since high precision (±1 μm ) production die-placement tools are commercially available for flip-chip bonding, it is expedient that such tool be used in order to place the optoelectronic chip precisely in position relative to the fiber. In that case, a submount fabricated with an etched hole for flip-chip mounting would be used, as in FIGS. 4 and 4 b . The procedure employed is illustrated in FIGS. 5 a  and  5   b.    
     First, as shown in FIG. 5 a , the fiber is threaded into the submount hole from the back side. It is pushed against a special temporary stop so that the fiber end is not flush with the front surface of the submount, but is slightly retracted therefrom (approximately 10-20 μm, for example). The fiber is then fixed in place with a suitable adhesive, such as UV-crosslinked epoxy, and the stop is then removed. It is noted that it is not necessary to align the fiber with respect to the sides of the hole; it should merely be positioned roughly near the center of the hole. 
     A circle is an ideal object for recognition and centroid determination by a machine vision system. The fiber ends present just such an object for the vision system of the die-placement tool. For example, either the outside diameter of the fiber could be used as a fiducial mark, or alternatively, visible light could be directed into the fiber and the illuminated fiber core could be used for that purpose. It may be assumed that corresponding fiducial marks which are keyed to the position of the optically active areas are fabricated on the optoelectronic chips. The flip-chip die-placement tool then aligns those fiducial marks to the image of the fiber end with excellent accuracy, and die bonds the chip on the submount, as shown in FIG. 5 b.    
     (c) Application to Arrays 
     The foregoing description has been directed to the alignment of a single fiber to a single optoelectronic elements. However, these concepts may be readily extended to the alignment of an array of fibers to a chip containing an array of optoelectronic elements. 
     If the fiber-guide concept as described hereinabove is used in this case, it is possible to insert and fix fibers sequentially into the various fiber guides of the array chip. However, it is more cost-effective to insert an array of fibers simultaneously into the array of fiber guides. If, as is customary, the fibers are captured in a holder (such as an array of V grooves precision etched in silicon), the fiber-fiber spacing can be held to within ±1 μm; the optoelectronic elements on the chip are also spaced accurately since they are made by photolithographic means. Consequently, it is then necessary only to guide the fibers at either end of the fiber array into their respective fiber guides inasmuch as the remaining fibers will automatically be correctly inserted. For this purpose, it is necessary to fabricate the fiber array in a manner such that the fibers protrude slightly from the end of their holder. Since the protrusion need only be a fraction of a mm, the fibers will be stiff enough to maintain their relative positions. Alternatively, the flip-chip concept, as described hereinabove, may be used for arrays. 
     While it is apparent that the invention herein disclosed is well calculated to fulfill the objects stated above, it will be appreciated that numerous modification and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.