Subassembly for optoelectronic devices

Abstract: A subassembly (10) for use in packaging an optoelectronic device (e.g., LED or photodiode 11) includes a semiconductor (e.g., silicon) base (22) and lid (24) having a variety of etched features (e.g., grooves, cavities, alignment detents) and metalization patterns (e.g., contacts, reflectors) which enable the device to be reliably and inexpensively mounted on the base and coupled to the fiber (20).

Description: This invention relates to lightwave devices and more particularly to a subassembly used in packaging optoelectronic devices such as photodiodes and light emitting diodes (LEDs). Conventional packages for discrete optoelectronic devices typically are made out of a variety of dissimilar materials, such as metal, glass and ceramic, and involve relatively complicated manipulation of components during assembly. U. S. Patents No. 4,357,072 (LED), 4,119,363 (laser) and 4,233,619 (photodetector) are illustrative. Assembly involves manipulating components in three dimensions in order to place the components in the desired locations; for example, alignment of a device to a substrate, alignment of a fiber to a ferrule, alignment of a ferrule to a package, and finally alignment of the package to the device. These alignment steps depend upon fairly specialized, expensive equipment. In general, therefore, z-axis assembly is not possible in those designs; that is, one cannot simply move the components along a z-axis perpendicular to the substrate and place them on the substrate without further orientation being required in the x-y plane. In addition, because the packages include dissimilar materials, great care is required to insure that the thermal-expansion coefficients are adjusted in such a way that the alignments are stable even though the ambient temperature changes. Instability, of course, leads to mechanical movement of the components and consequently to a decrease in optical coupling between, for example, the device and the fiber. Often this situation leads package designers to compromise between thermal sinking and stability criteria. At the very least, it increases cost. For many expensive applications, notably long haul transmission systems, the relatively high cost of manufacturing such packages can be tolerated. However, for some cost sensitive, high volume applications, such as lightwave systems between central switching offices or remote-terminals and subscribers' homes (known as the subscriber loop), the high cost of such packages renders them unsuitable. Thus, it would be desirable to have a packaging scheme which (1) uses, as much as possible, commercially available equipment for the production of the pieceparts, for the alignment of the parts, and for closing of the package; and (2) which accommodates different size packages for different applications without requiring extensive retooling or new assembly techniques. For example, it would be desirable, on the one hand, to have a subassembly which fits inside of a dual-in-line package (DIP) header and is adapted to a 125 µm diameter fiber and, on the other hand, to have a different subassembly which is adapted to a larger diameter fiber and is surface mounted on a printed circuit (PC) board. Normally, the fabrication of these two designs would require retooling which is undesirable for reasons of cost and commonality. Reference DE 3,543,558 teaches an optoelectronic coupling arrangement including an optical waveguide (i.e. optical fiber) disposed in groove formed in a substrate. The substrate further includes a deflection component for redirecting the light propagating along the optical waveguide out of the plane of the substrate and into a light sensitive device positioned above the substrate. According to the present invention there is provided a subassembly as defined in claim 1. One aspect of the invention is the use of a single crystal semiconductor such as silicon as the basic material from which the subassembly for packaging optoelectronic devices is constructed and to exploit many of the well-known techniques for shaping silicon pieceparts and for depositing metals thereon. The use of silicon in this way will be referred to as "silicon optical bench" technology. The maturity of silicon technology means several things: first, there is available a large body of silicon processing techniques which, therefore, do not have to be specifically developed; second, there are available numerous pieces of commercial equipment which have been already cost-reduced by years of high volume manufacturing: and, third, using these techniques and equipments, various shapes in silicon pieceparts can be realized by relatively simple and inexpensive photolithographic processes, which do not require large volume to render them cost effective. In accordance with the invention, the subassembly includes an optoelectric device mounted on a silicon base and the two following features: (1) A groove is located in the base for guiding an optical fiber and a reflector is formed on the end face of the groove. The device is placed over the reflector which directs light between the device and the fiber. Positioning the device over the reflector is the only active alignment step required in the x-y plane. All other alignments are automatically achieved by the geometry of the pieceparts; (2) Electrical contacts are disposed on the base with one of the contacts overlapping the end face of the groove to form the reflector. The subassembly of the present invention may also have one or more of the following additional features : (3) A silicon lid is disposed on the base with at least one dimension of the lid being less than the corresponding one of the base so that the contacts protrude from under the lid to the exterior of the subassembly; (4) The lid has a cavity for receiving the device and a groove, aligned with the groove in the base, to form a channel in which the coated part of the fiber is positioned; (5) Detents are provided in the base and lid for aligning them to one another; (6) Portions of the exteriors of the base and lid are metalized to provide a ground shield and a place to make electrical contact to the rest of the package. In another embodiment for devices, such as photodiodes, which are passivated by a conformal coating, an aperture in the lid communicates with the cavity and the coating is injected therethrough. The invention, together with its various features and advantages, can be readily understood from the following more detailed description taken in conjunction with accompanying drawings, in which the various figures are schematic and hence are not drawn to scale in the interests of clarity. FIG. 1 is an isometric, cut-away view of a package for an optoelectronic device; the package includes a subassembly 10 in accordance with an illustrative embodiment of the invention. FIG. 2 is an exploded view of FIG. 1 showing the illustrative subassembly 10, a lead frame 12, a DIP header 14, and a cover 16. FIG. 3 is an expanded view of a cross-section of the optical subassembly 10 of FIG. 1 depicting a photodiode 11 therein. FIGs. 4-7 show isometric views of the illustrative subassembly 10 in various stages of fabrication. With reference now to FIGs. 1-3, subassembly 10 in accordance with one embodiment of the invention is shown enclosed within a dual-in-line package (DIP). Other package designs, such as molded ones, are also suitable, however. In the DIP configuration the subassembly 10 is mounted on a lead frame 12 which is inserted in a DIP header 14. A cover 16 has a slot 18 to permit an optical fiber 20 to extend from the subassembly 10 to the exterior of the package. Fiber 20 permits lightwave signals to be transmitted to and/or from an optoelectronic device (e.g., photodiode 11) within the subassembly. The subassembly 10 is constructed in accordance with the invention from a single crystal semiconductor, preferably silicon in order to exploit the mature silicon technology from the standpoint of processing and equipment availability, as discussed earlier. In addition, the design reduces to one the number of alignment steps which have to be performed in the x-y plane. Thus, the invention is advantageously very close to the desiderata of z-axis alignment. More specifically, FIGs. 4-7 show subassembly 10 for a photodiode 11 including a single crystal silicon base 22 on which the photodiode is mounted, a single crystal silicon lid 24, and an optical fiber 20 disposed in a channel formed along the interface between the base and lid so that it is optically coupled to the photodiode. The subassembly 10 is characterized by one or more of the following features. (1) Anisotropically-etched tandem grooves (30 and 32 in the base and 34 in the lid) provide strain relief for and alignment of fiber 20. Groove 30 in the base is aligned with groove 34 in the lid so as to receive the coated portion 21 of fiber 20, and the width of groove 34 is slightly smaller than that of groove 30 so that portion 21 fits snugly and provides strain relief. Groove 32, on the other hand, is formed in the base in tandem with groove 30 but has a considerably smaller width so as to receive and align the bare end portion 23 of fiber 20. The use of tandem grooves is preferred where the shape and structural strength of the fiber coating (e.g., urethane) are not sufficiently well controlled in different environmental conditions (e.g., humidity) that the coating can be relied on for alignment purposes. On the other hand, some fiber coatings (e.g., polyimide) are suitable for alignment purposes; and, in those cases, neither the tandem groove arrangement in the base nor the groove in the lid need be utilized; rather the base may simply have a groove for holding the coated portion 21 of the fiber against the bottom of the lid. (2) After the grooves are etched, the base is oxidized (e.g., 1-10 µm of SiO₂ is formed) and metalized, and standard photolithography is utilized to pattern electrical contacts 40 and 42. The oxide layer (not shown) prevents the contacts from being short circuited through the silicon and reduces their mutual capacitance. As shown in FIG. 4, contact 40 overlaps the end face of groove 32 and forms a turning mirror of reflector 44. When the major surface of the base is a (100) crystallographic plane, the end face is a (111) crystallographic plane and hence reflector 44 is oriented at a 54.7° angle. The latter serves both as a mechanical stop for the fiber and as a director of light emanating from the end of fiber 20. The reflected light is thus made incident on the photosensitive area of photodiode 11. Even though mirror 44 is not at a 45° angle, it is adequate to couple light into the photodiode and to enable the axis of the fiber and that of the photodiode to be aligned to about ± 1 µm (where ± 5 µm accuracy is sufficient). (3) The photodiode is illustratively an InP/InGaAs back-illuminated design of the type described by O. K. Kim in U. S. Patent No. 4,608,586; that is, the device is illuminated through an opening in the metalization on its transparent substrate, thereby to permit ingress of light into the photosensitive area of the photodiode. The substrate metalization of the photodiode is bonded to contact 40, and the photodiode is oriented in the x-y plane so that its photosensitive area is over the mirror 44. Advantageously, this step is the only active alignment step required in the assembly procedure. After alignment is accomplished, the top contact of the photodiode is connected to contact 42 via a wire bond 46, as shown in FIG. 5. (4) The dimensions of the base, lid and contacts are mutually adapted so that the pads at the ends of the contacts 40 and 42 extend out from under the lid, as shown in FIGs. 6 and 7, thereby enabling wire-bonds to be made to the lead frame. One such bond 48 is shown in FIG. 1. (5) The lid and base are aligned relative to one another by means of detents 50. Illustratively, these detents are small pyramidal-shaped holes etched into the lid and base with a ball bearing in each hole in the base to facilitate alignment by merely snapping the base and lid together. The balls may be fabricated from a number of materials such as metal, sapphire or tungsten carbide. This technique has a relatively large capture range because the balls are curved and, therefore, tend to readily guide a slightly misaligned lid into alignment. In addition, instead of aligning lids and bases one at a time, one can take advantage of the fact that the bases are cut from a wafer and that prior to separating the bases, the photodiodes can be mounted thereon, tested and burned in before the lids are put in place. Or, the bases may be cut into 1 x N strips before the photodiodes, N in number, are mounted. A similar 1 x N strip of lids can then be aligned to the strip of bases using, for example, only a pair of alignment detents, one at each end of the strips rather than a pair for each individual lid and base. (6) The lid has a cavity 26 and a pair of apertures 52 and 54, as shown in FIG. 7. When the lid is in place, the photodiode 11 is located in the cavity and funnel-shaped aperture 52 permits a conformal coating (e.g., silicone for passivation) to be inserted therethrough (and then cured) to cover the photodiode. Likewise, aperture 54 permits epoxy or other adhesive to be inserted to secure the position of the fiber 20 in the subassembly. Where, however, passivation is not required (or a hermetic design is), aperture 52 may be omitted. Likewise, where the fiber needs to slide in and out of the subassembly (in a plug-in design, for example), the epoxy/adhesive and aperture 54 may be omitted. (7) The top of the lid and the bottom of the base are metalized (not shown) to provide a ground shield. For adhesion purposes, the silicon may be oxidized before the metal is deposited. As shown in FIG. 1, the metalized bottom of the base contacts lead frame 12, and the top of the lid is connected thereto by wire bond 49. Subassemblies according to FIGs. 1-7 have been fabricated using silicon optical bench technology with the silicon pieceparts (base, lid) measuring about 3 mm wide by 4 mm long by 750 µm thick. The anisotropically etched V-grooves 30, 32 and 34 were adapted to receive single mode glass fibers 20 having a coated portion 21 with about a 250 µm diameter and a bare portion 23 with about a 125 µm outside diameter. The oxide on the silicon base was about 10 µm thick. The contacts 40 and 42 and mirror 44 were Ti-Pt-Au multilayers (Cr-Au is also suitable) with Au the outermost layer. The photodiode 11, which had an active area of about 75 µm, was bonded to contact 40 with a eutectic solder (Au/Ge or Au/Sn) which was evaporated onto the substrate side of the photodiode. Because the silicon base is extremely flat, very thin (e.g., 3 µm) layers of solder could be used thereby reducing expense associated with the cost of a separate preform of eutectic solder. From a convenience standpoint, the fiber should not be secured into the subassembly too soon during the fabrication sequence, otherwise manipulation of the subassembly may be cumbersome. Thus, it is preferred that, after the base, photodiode and lid are in place, but before the conformal coating is applied, the subassembly is bonded to the lead frame with epoxy or with a lower melting point solder than used for the photodiode. After wire bonds are made to the photodiode and to the ground shields, the fiber is inserted into the V-grooves of the subassembly and epoxied in place. It is apparent that the lid serves multiple purposes including: protecting the device during molding operations associated with encapsulating the subassembly in a package; electrically isolating the device; containing epoxy and the conformal coating, if necessary; and guiding the optical fiber. In operation, photodiodes packaged in this fashion are relatively inexpensive, have reasonably high yields and responsivities of about 0.8-0.9 A/W. No failures were observed when the packages were cycled between -40°C and +85°C in ambient humidity. The frequency response of the package depends on a number of factors, principally the oxide thickness, the area of the contacts and the length of the leads. When property designed, a frequency response of about 200 MHz was measured. It is to be understood that the above-described arrangements are merely illustrative of the many possible specific embodiments which can be devised to represent application of the principles of the invention. Numerous and varied other arrangements can be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention. In particular, although the subassemblies of FIGs. 1-7 were described in terms of light in a fiber being detected by a photodiode, they are also useful for coupling light from an LED into the fiber. However, when doing so, the divergence of the light emission from the LED affects the coupling efficiency into the fiber, and the relatively high current carried by the LED requires adequate heat sinking. Finally, it will be appreciated that although today's technology suggests that silicon is the preferred material from which the principal pieceparts of the invention are made, it is possible that different circumstances (e.g., system specifications and cost considerations) might render it desirable to utilize a different semiconductor (e.g., Ge, GaAs).

Claims: An optoelectronic device subassembly (10) comprising a semiconductor base (22) having a major surface, a first groove (32) in said major surface adapted to receive a portion of an optical fiber (20), said first groove having side walls and an end face formed by crystallographic planes which are oblique to said major surface, said groove end face forming a reflector and an optoelectronic device (11) being disposed over said reflector, CHARACTERIZED BY an insulative layer formed on at least a portion of said major surface, and metal contacts (40,42) disposed on said layer, at least one of said contacts (40) overlapping said end face and forming said reflector (44), said device being connected to said contacts, so that said reflector directs light between said device and said fiber. The subassembly of claim 1 wherein said base has a second groove (30) arranged in tandem with said first groove and extending to the exterior of said subassembly, said fiber having a larger diameter portion (21) which fits into said second groove and a narrower diameter portion (23) which fits into said first groove. The subassembly of claim 2 wherein the length of said grooves permits the end of said fiber to abut said reflector. The subassembly of claim 1 further including a semiconductor lid (24) disposed on said base, said lid having a major surface and a cavity (26) therein for receiving said device, and further having a third groove (34) therein aligned with said first groove, thereby forming an elongated channel for receiving said fiber. The subassembly of claim 4 wherein said lid has an opening (52) in communication with said cavity and a conformal coating, inserted through said opening, covers said device. The subassembly of claim 4 including said fiber attached thereto and wherein said lid has an opening (54) in communication with said elongated channel and an adhesive, inserted through said opening, secures said fiber to said subassembly. The subassembly of claim 4 wherein exterior major surfaces of said base and lid are metalized. The subassembly of claims 1-6 or 7 wherein said semiconductor constitutive of said base comprises single crystal silicon, said major surface is a (100) crystallographic plane and said end face is a (111) crystallographic plane. The subassembly of claims 1-6 or 7 wherein the optoelectronic device is a photodiode. The subassembly of claim 1 further including (FIG.7) a semiconductor lid having a cavity, said lid being disposed over said base so that said device is positioned in said cavity, at least one dimension of said lid parallel to said surface being less than the corresponding dimension of said base, so that segments of said contacts (40,42) extend from under said lid to the exterior of said subassembly, and said subassembly having channel means for receiving an optical fiber for carrying light between said device and the exterior of said subassembly. The subassembly of claim 10 wherein said lid has an opening (52) in communication with said cavity, and a conformal coating, inserted through said opening, covers said device. The subassembly of claim 10 including said fiber attached thereto and wherein said lid has an opening (54) in communication with said channel means and an adhesive, inserted through said opening, secures said fiber to said subassembly. The subassembly of claim 12 wherein said channel means comprises a groove (30) in said base and a groove (34) in said lid, said grooves facing, and being aligned with, one another and having side walls which are crystallographic planes. The subassembly of claim 13 wherein said channel means comprises a reflector (44) and a third groove (32) in tandem with the groove in said base, said third groove having a crystallographic end face on which said reflector is disposed, said device being positioned over said reflector which directs light between said device and said fiber. The subassembly of claims 10-13 or 14 wherein said base and lid comprise single crystal silicon.

Inventors: BLONDER GREG E, JOHNSON BERTRAND HAROLD, BLONDER, GREG. E., JOHNSON, BERTRAND HAROLD
Applicants: AT & T CORP, AT&T CORP.
Application year: 1989
Main classification: G02B6
IPC codes: H01L23, H01L33, G02B6, H01L31