Self-aligned optical subassembly and method of forming such subassembly

Abstract: An optical subassembly is disclosed which provides self-alignment between an active optical device (14) and associated optical elements (16,82). In particular, the subassembly comprises a first substrate member (12) which is utilized to support the active optical device. A separate plurality of substrate members (18) is used to form a support for the associated optical elements (i.e., fiber, coupling lens or both), where the separate members are stacked, one on top of the other, and an aperture (20) through the center thereof utilized to support the optical elements. The aperture may be formed to include a mechanical stop (70) suitable for fixing the the location of an optical fiber endface. Alignment fiducials (22) formed on the device support member and the stack provide self-alignment therebetween, as well as physical attachment of the support member to the stack. The various substrate members may comprise any suitable material including, but not limited to, silicon, metal, or molded plastic.

Description: Background of the Invention Technical Field The present invention relates to an optical subassembly and, more particularly, to an optical subassembly which provides alignment between an active optical device and associated optical elements. The present invention also relates to a method of forming such optical subassembly. Description of the Prior Art In the realm of optical device packaging, there is beginning to develop a number of assemblies which utilize a single crystal semiconductor material (such as silicon) as the support structure for the various optical devices. Often referred to as "silicon optical bench" technology, the utilization of silicon may result in a significant cost saving in optical packaging over some of the more esoteric materials which have been used in the past. More importantly, silicon processing technology has advanced to the stage where a number of relatively standard procedures (e.g., oxidation, metallization, etching - isotropic or anisotropic) may be utilized to facilitate attachment of the devices to the support member, as well as alignment therebetween. Further, it is possible to form optical waveguiding structures directly in/on a silicon substrate, resulting in the ability to form a completely operable optical subassembly in silicon. An exemplary utilization of silicon in the formation of a subassembly for optoelectronic devices is disclosed in U. S. Patent 4,945,400 (Blonder et al.) issued July 31, 1990 and assigned to the assignee of record in this application. In general, Blonder et al. disclose a subassembly including a semiconductor (e.g., silicon) base and lid including a variety of etched features (e.g., grooves, cavities, alignment detents) and metallization patterns (e.g., contacts, reflectors) which enable the optoelectronic device to be reliably and inexpensively mounted on the base and coupled to a communicating optical fiber. In particular, Blonder et al. disclose an arrangement wherein the optoelectronic device (e.g., LED) is disposed within a cavity formed by a lid member and the communicating fiber is positioned along a groove formed in a base member. A reflective metallization is utilized to optically couple the device to the fiber. Therefore, positioning of the device over the reflector is the only active alignment step required to provide coupling. Any remaining alignments are accomplished utilizing fiducial features formed in the base and lid members. Although the Blonder et al, subassembly represents a significant advance in the field of silicon optical bench packaging, a need remains for providing an arrangement which does not require active alignments. In particular, the provision of a completely passive optical packaging arrangement is considered both more reliable and less expensive than virtually any alternative requiring active alignment between components. EP-A-0 413 489 and EP-A-0 213 856 disclose alternative mechanical arrangements for forming an optical package. The former reference discloses a hermetic package for coupling a fiber to an optical device, using a first silicon substrate to hold the optical device and a second silicon substrate for guiding the bared endface of the optical fiber. The silicon substrate includes alignment fiducials to facilitate registration between the optical device and the fiber. The latter reference discloses, in one embodiment, alignment fiducials in the form of beads and wells. WO 87/01465 discloses an assembly for coupling an active optical device and an optical fiber. composed of a stack of platelets pierced each with a viahole aligned with the other ones to provide a longitudinal aperture, wherein an optical fiber is received in said aperture whilst an active optical device is mounted on the stack outer surface, overlying the aperture. To ensure the alignment of all the viaholes. a piece of wire is inserted in said viaholes during the assembling and the securing together of the platelets. Summary of the Invention According to the present invention. there is provided a subassembly as defined in claim 1 and a method as defined in claim 8. The problems remaining in the prior art are addressed by the present invention which relates to an optical subassembly and, more particularly, to an optical subassembly which provides alignment between an active optical device and associated optical elements, as further defined by the appended claims. Brief Description of the Drawing FIG. 1 illustrates an exemplary optical subassembly formed in accordance with the teachings of the present invention; FIG. 2 illustrates a plan view of an exemplary optical device support member; FIG. 3 illustrates a selected cross-section view of the member of FIG. 2; FIG. 4 illustrates a plan view of exemplary member utilized to form the stack; FIG. 5 illustrates an exemplary stack utilizing a plurality of members as illustrated in FIG. 4; FIG. 6 illustrates an alternative stack arrangement; FIGs. 7-11 illustrate an exemplary fabrication process for forming a fiber mechanical stop within an exemplary silicon stack member; and FIG. 12 illustrates an alternative embodiment of the present invention incorporating a spherical lens and fiber mechanical stop within the subassembly. Detailed Description FIG. 1 illustrates, in an exploded view, an exemplary optical subassembly 10 formed in accordance with the teachings of the present invention. As shown, optical subassembly 10 comprises a first member 12 which is utilized as the support member for an active semiconductor optical device 14. An optical fiber 16 is illustrated in FIG. 1 as being coupled to optical device 14. In accordance with the teachings of the present invention, optical fiber 16 is mechanically supported by a stack of members 18, disposed as shown in FIG. 1. Exemplary optical subassembly 10 utilizes a pair of members 18 1 and 18 2 to form stack 18. Optical fiber 16 is inserted through an aperture 20 within stack 18, where aperture 20 comprises a pair of viaholes 20 1 and 20 2 formed completely through each member 18 1 and 18 2 , respectively. Stack 18 is aligned with, and mated to, support member 12 by a plurality of alignment fiducials 22. In the exemplary embodiment of optical subassembly 10, alignment fiducials 22 comprise a first plurality of detents 24 formed in surface 13 of member 12 and a second plurality of detents 26 formed in surface 17 of stack 18, where detents 26 are formed to align with associated detents 24. A plurality of spherical members 28 are disposed between first plurality 24 and second plurality 26 to provide mechanical attachment between support member 12 and stack 18. Therefore, since the detents are accurately formed in a pattern predetermined to provide the required alignment, optical fiber 16 will be self-aligned to active optical device 14 when stack 18 is fixed to support 12, as indicated by the directional arrows in FIG. 1. A plan view of an exemplary optical device support member 12 is shown in FIG. 2. In this particular embodiment, optical device 14 is illustrated as disposed within a recess 30 (see FIG. 3) formed in top surface 13 of member 12. Alternatively, optical device 14 may simply be attached to the planar top surface 13 of support member 12. A first metal layer 32 is formed on surface 13 and coupled to device 14 to provide a first electrical contact to the optical device. A second metal layer 34 is formed to provide the remaining contact. A wirebond 36 is used to provide the electrical connection between metal layer 34 and the top side 15 of optical device 14. Subsequent to the formation of metal layers 32,34, a leadframe section (not shown) may be attached to metal layers 32,34 of support member 12 to provide the final electrical leads associated therewith. A set of three alignment fiducials 24, in the form of pyramidal detents, are illustrated in exemplary support member 12 of FIG. 2, where the detents are arranged such that a stable mechanical attachment is achieved. It is to be understood that any suitable number of such alignment fiducials, including kinematic designs, may be utilized when practicing the present invention. FIG. 3 contains a selected cross-section view in perspective of exemplary support member 12 as depicted in FIG. 2. Shown in particular in this view is exemplary recess 30, formed to a predetermined depth d 1 below surface 13 of member 12. Depth d 1 may be chosen such that optical device 14, upon subsequent attachment, remains fully recessed below surface 13. Detents 24 are shown as formed to include a surface width w and a depth d 2 sufficient to accommodate an alignment sphere of known proportions. When utilizing silicon in the formation of member 12, recess 30 and detents 24 may be formed by etching, using appropriate masking of top surface 13. Alternatively, when utilizing a plastic, these features may be included in the mold, or formed as a secondary operation upon the molded piece part. In accordance with the self-aligning feature of the present invention, exemplary alignment fiducials 24 are formed at a predetermined distance D from the center of recess 30. Optical device 14 may then be accurately positioned within recess 30, with respect to first metal layer 32, using a liquified amount of solder which will conformally coat the underlying pattern of first metal layer 32. The surface tension of the liquid material will cause the optical device to center itself and thus become aligned with metal layer 32. Therefore, by the appropriate choice of distance D and the subsequent coaxial placement of the associated stack alignment fiducials (see FIG. 4), optical alignment will be achieved. A plan view of an exemplary stack member 18 i is illustrated in FIG. 4. A viahole 20 i is shown as being formed completely through member 18 i . When utilizing silicon as the member, both the top surface 17 and bottom surface 19 (visible in FIG. 5) may be etched to form the viahole. The utilization of a conventional anisotropic etchant (EDP or KOH, for example) may result in the formation of an unwanted knife-edge 23 (see FIG. 5) within viahole 20 i . However, such an etch may be required to form the desired physical design of the alignment detents. A set of three such exemplary detents 26 are illustrated as formed in top surface 17 of member 18 i , where detents 26 are formed to align with detents 24 of member 12 upon mating. FIG. 5 illustrates an exemplary stack 18 comprising separate members 18 1 , 18 2 , 18 3 . A plurality of knife-edges 23, which may be formed when performing a single step, anisotropic etch upon silicon support members, are clearly visible in this view. The individual members forming stack 18 may be joined using the technique described above for joining stack 18 to member 12. That is, a plurality of detents 42 may be formed in each member 18 i , as shown, with a plurality of spherical members 44 used to physically align the separate pieceparts. Thermionic or anodic bonding may be used to fix this attachment. An alternative stack arrangement, which avoids the formation of knife-edges, may be utilized in accordance with the teachings of the present invention. FIG. 6 illustrates a top plan view of an exemplary stack member including a viahole 50, etched in <110> silicon so as to form opposing parallel faces 52, 54 and opposing tapered faces 56, 58. A pair of such members may be stacked, with one member rotated orthogonal to the other such As shown, each of the individual members forming the stack is fabricated to include a viahole 50 comprising opposing parallel faces 52, 54 and opposing tapered faces 56, 58. A pair of such members may then be stacked, with one rotated orthogonal to the other, as shown by the via 50' illustrated in phantom in FIG. 6. The orthogonal displacement of the pair of members results in the such intersection of parallel faces 52, 54, 52' and 54' of viaholes 50 and 50' forming a "box", as indicated by the heavy line in FIG. 6. A fiber 16 may then be constrained within the dimension of the box as outlined by these parallel surfaces without the concern of a knife-edge formation. Alternatively, appropriate etching and masking steps may be used to avoid the formation of knife edges. Additionally, the fabrication process may be used to form a mechanical stop for a fiber endface within the stack. FIGs. 7-11 depict a particular series of such steps utilized with an exemplary silicon stack member 18. Referring to FIG. 7, oxide layers 60 and 62 are first grown over top and bottom surfaces 17 and 19 of member 18, respectively. Oxide layers 60 and 62 are subsequently patterned and etched to expose the underlying silicon in windows 64 and 66, where window 64 comprises a width w 1 somewhat less than the width w 2 of window 64, the difference in width calculated to form the desired fiber mechanical stop (as will be evident from the subsequent processing operations). The structure as depicted in FIG. 7 is then etched for a predetermined period of time so as to form openings 65 and 67, as shown in FIG. 8, where each is formed to a depth d, leaving a relatively small thickness t of silicon therebetween. Referring to FIG. 9, a first oxide layer 68 is subsequently grown over top surface 17, as well as etched opening 65, of member 18. Similarly, a second oxide layer 70 is grown over bottom surface 19 and etched opening 67. As is well-known in the silicon processing art, the formation of a thermal oxide layer involves the inward movement of the oxide at the silicon/oxide interface, as the underlying silicon consumed. Advantageously, the removal of the silicon during this process results in rounding knife edges 23 (see FIG. 3) and forming rounded corners 69,71 as shown in FIG. 9. The rounded profiles of these areas thus reduces the problems (e.g., damage to fiber) associated with knife edges. Oxide layer 70 is subsequently removed, and via opening 67 is further etched until oxide layer 68 is reached, where oxide 68 will act as a natural etch stop. The structure at this point in the process is illustrated in FIG. 10. Upon removal of the remaining oxide material, stack member 18 is configured as shown in FIG. 11. As shown, a mechanical stop 74 for a fiber 16 is formed at the interface between the top and bottom etched regions, where the size of opening 76 is controlled (by controlling, for example, window widths, etchants, various etch parameters) so as to allow the core region of a fiber 16 to be coupled to the active area of an associated optical device (not shown). As mentioned above, a spherical coupling lens may be included in a subassembly formed in accordance with the teachings of the present invention. FIG. 12 illustrates an exemplary embodiment 80 of the present invention including such a lens. In particular, subassembly 80, as shown in FIG. 12, includes a lens 82 resting upon top surface 84 of a fiber stack 86, where the upper most member 86 1 of stack 86 is formed to include a recess 90 sufficient to capture lens 82. Additionally, member 86 1 is processed to include a fiber stop 92, using the oxide growth and etch steps outlined above in associated with FIGs. 7-11. Therefore, an optical fiber 94 may be inserted through an aperture 96 within stack 86 and the endface 98 of fiber 94 will be fixed by the location of mechanical stop 92. In an alternative arrangement, lens 82 may be positioned within aperture 96, below top surface 84, as shown in phantom in FIG. 12. It is to be understood that the above-described embodiments of the present invention are exemplary only. For example, the various substrate members described above may be formed from a plastic material which is suitably molded utilizing either injection-molded or transfer-molded technologies. Some applications, particularly where cost is of a concern, plastic may be the material of choice. Indeed, the preferred alternative may utilize a silicon support member for the active device (for thermal management purposes) and a plastic stack (for cost reduction purposes). Further, various types of alignment fiducials, other than the above-described detent/sphere combination, may be used. For example, alignment grooves and ridges may be formed in the adjacent surfaces and interlocked to provide the physical attachment.

Claims: An optical subassembly (10;80) for aligning an active optical device (14) and an optical fiber (16; 94), comprising a first substrate member (12) including an outer major surface (13) having a location for supporting an active optical device (14), said first member further including a first plurality of alignment fiducials (24) formed in said outer major surface at predetermined locations with respect to said location for the active optical device; and a plurality of second substrate members (18i; 86i) disposed to form a stack (18; 86) defined as including an outer major surface (17); each of said second members (18i) including a viahole (20i; 50, 50') provided therethrough and a set of stack alignment fiducials (42) formed in each major surface thereof to be joined with an confronting major surface of an adjacent second member when the stack is to be formed, each of said sets of stack fiducials (42) being positioned in the corresponding major surface at the same predetermined locations with respect to the corresponding viahole so that, as the second members are joined to form the stack, the stack fiducials (42) align and mate to provide for the alignment of the viaholes so as to form through the stack a longitudinal aperture (20; 16) for receiving an optical fiber (16; 94); said stack (18, 86) further comprising a second plurality of alignment fiducials (26), formed in said outer major surface (17) and positioned with respect to said longitudinal aperture at the same predetermined locations as those at which the first alignment fiducials (24) of the first member (12) are positioned with respect to the location for the active optical device, so that when the stack (18; 86) is mated with the first member (12) via the confronting outer major surfaces (13 and 17), said second plurality of alignment fiducials (26) of said stack align and mate with said first plurality of alignment fiducials (24) of said first member. thereby achieving optical self-alignment between the location for supporting the active optical device (14) and the longitudinal aperture (20; 16) for receiving the optical fiber (16; 94). A subassembly as defined in claim 1 wherein the first alignment fiducials (24) comprise a plurality of detents (26) formed in the outer major surface (13) and the second alignment fiducials (26) comprise a like plurality of detents (26) formed in the outer major surface (17), and wherein a plurality of spherical members (28) may be disposed therebetween to provide mechanical attachment and alignment. A subassembly as defined in claim 1 or 2 wherein the stack (18; 86) is formed to further support a spherical lens member (82) between the active optical device (14) and the optical fiber (16; 94), said spherical lens member being either supported in the outer major surface of the stack or within the stack aperture so as to be aligned with the active optical device location on the first substrate member. A subassembly as defined in claim 1,2 or 3, wherein the stack (18; 86) is formed to include an annular mechanical stop (70; 92) within the stack aperture. said mechanical stop suitable for fixing the location of the optical fiber within the stack, and/or the first substrate support member (12) is formed to include a recess (30) below the outer major surface (13), said recess for supporting the said optical device. An optical subassembly as defined in any one preceding claim. wherein each substrate member comprises silicon, and the first alignment fiducials (24) are obtainable by etching the outer major surface of said first member (12), the plurality of viaholes and alignment fiducials of the stack (18, 86) being obtainable by etching the appropriate surfaces of the separate silicon substrates forming said stack, said etching optionally including anisotropically etching the major surface of the stack and member. An optical subassembly as defined in claim 5, wherein a selected silicon substrate member of the plurality of members comprising the stack includes a mechanical stop (70; 92) within the associated viahole, the mechanical stop being obtainable by etching and suitable for fixing the location of an optical fiber within the stack aperture. An optical subassembly as defined in any one preceding claim, wherein at least one of the substrate members is of plastic, the latter being optionally injection-molded plastic members, or transfer-molded plastic members. A method of forming an optical subassembly (10; 80) for aligning an active optical device (14) and an optical fiber (16; 94), comprising the steps of: a) providing a first substrate member (12), said first member including an outer major surface (13) having a location for supporting an active optical device (14); b) forming a first plurality of alignment fiducials (24) in the outer major surface of said first member. at predetermined locations with respect of the location thereof for supporting the active optical device: c) providing a plurality of second substrate members (18i; 86i) adapted for being stacked upon one another, each second member including two opposed major surfaces; d) forming a viahole (20i; 50, 50') completely through each member of said plurality of second substrate members provided in step c); e) forming a set of stack alignment fiducials (42) in each major surface of each second member to be joined with an confronting major surface of an adjacent second member when the stack is to be formed, each of said sets of stack alignment fiducials (42) being positioned in the corresponding major surface at the same predetermined locations with respect to the corresponding viahole; and forming a second plurality of alignment fiducials (26) in the major surface (17) of a selected member (18 1 ; 86 1 ) of said plurality of second substrate members intended to form an outer major surface in the stack (18) to be formed, said second plurality of alignment fiducials being positioned with respect to the viahole (20 1 ) of said selected member at the same predetermined locations as those at which the first plurality of alignment fiducials (24) of the first member (12) formed in step b) are positioned with respect to the location for the optical device; and f) stacking the plurality of second substrate members upon one another so that the sets of stack fiducials (42) align and mate and the plurality of viaholes align with one another and form a longitudinal aperture (20; 16) therethrough for receiving an optical fiber (16; 94); whereby, as the stack (18; 86) is mated with the first member (12) via the confronting outer major surfaces (13 and 17), said second plurality of alignment fiducials (26) of said stack align and mate with said first plurality of alignment fiducials (24) of said first member, and opical self-alignment between the location for supporting the active optical device (14) and the longitudinal aperture (20; 16) for receiving the optical fiber (16; 94) is achieved. The method according to claim 8, wherein the method comprises the further steps of: g) attaching a semiconductor active optical device (14) e.g. transmitting device) to the first substrate member; h) locating at least one optical device consisting of an optical fiber (16; 94) within the aperture formed in step f); and i) attaching the first substrate member to the stacked plurality of second members such that the first and second alignment fiducials are aligned and mated. The method according to claim 8 or 9 wherein in performing step (d), a mechanical fiber stop (70,92) is formed within the viahole of a selected second member by the following steps: d 1 ) oxidizing the two opposed major surfaces of said selected second member; d 2 ) patterning the oxide and etching to expose a recess of width w 1 in one of the major surfaces and a recess of width w 2 in the other major surface where the windows are coaxial and w 1 < w 2 ; d 3 ) reoxidizing the etched structure of step d 2 ; d 4 ) removing the oxide from said other major surface and etching through the recess until contacting the surface oxide layer at aid one major surface; and d 5 ) removing the remaining oxide, thereby providing said fiber stop at the intersection of the recesses. A method according to claim 8, 9 or 10, wherein the substrate members are of silicon and/or plastic. The method according to claim 9, wherein step (g) comprises g 1 ) forming a recess in the top major surface of the first substrate member; and g 2 ) disposing the semiconductor active optical device within the recess formed in step g 1 ). The method according to claim 9, wherein step (h) further comprises locating spherical lens (82) within the stack aperture.

Inventors: BENZONI ALBERT MICHAEL, DAUTARTAS MINDAUGAS FERNAND, BENZONI, ALBERT MICHAEL, DAUTARTAS, MINDAUGAS FERNAND
Applicants: AT & T CORP, AT&T CORP.
Application year: 1992
Main classification: G02B6
IPC codes: G02B6