Optical subassembly with passive optical alignment
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an optical subassembly and, more particularly, to an optical subassembly which utilizes passive alignment techniques to assemble the active device, lens and associated fiber in a unitary subassembly arrangement. 
     2. Background of the Invention 
     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 &#34;silicon optical bench&#34; 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 simple procedures (e.g., oxidation, etching, iso-tropic 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 subassembly in silicon. 
     An exemplary utilization of silicon in the formation of a subassembly for optoelectronic devices is disclosed in U.S. Pat. No. 4,945,400 issued Jul. 31, 1990 to G. E. Blonder et al. 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 a. 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 requires no active alignments to be performed. The provision of a completely passive optical packaging arrangement being considered both more reliable and less expensive than virtually any alternative requiring active alignment between components. 
     SUMMARY OF THE INVENTION 
     The need remaining in the prior art is addressed by the present invention relates to an optical subassembly and, more particularly, to an optical subassembly which utilizes passive alignment techniques to assemble the active device, lens and associated fiber in a unitary subassembly arrangement. 
     In accordance with a preferred embodiment of the present invention, a first substrate is utilized as a mount for the active optical device, with alignment fiducials formed at predetermined locations with respect to the placement of the active device. A second substrate is utilized as a lens holder (by virtue of an opening formed through the second substrate) and also includes alignment fiducials which will mate with the fiducials formed on the first substrate so as to provide mechanical joining of the two substrates and, advantageously, optical alignment between the lens and the active device (by virtue of accurate placement of the fiducials). An optical fiber receptacle is formed to mate with the structure supporting the aligned substrates and thus provide optical alignment between the fiber and the aligned active device and lens. 
     The substrates utilized in practicing the invention may be silicon or any other suitable material which may be processed to include the various features (i.e., alignment fiducials, openings) with the accuracy required to provide optical alignment. 
     An advantage of the arrangement of the present invention is that the substrate used as a device mount may be located within a conventional support structure used to provide the necessary electrical connections to the active device. 
     Other and further advantages of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Referring now to the drawings, where like numerals represent like parts in several views: 
     FIG. 1 illustrates an optical subassembly formed in accordance with the present invention; 
     FIGS. 2-4 illustrate various optical substrate configurations which may be utilized in accordance with the teachings of the present invention to provide optical alignment between the active device and coupling lens; 
     FIG. 5 illustrates a top view of an exemplary substrate which may be utilized as a mount for the active optical device; and 
     FIG. 6 illustrates, in an exploded view, an optical assembly utilizing the optical subassembly of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an exemplary optical subassembly 10 formed in accordance with the teachings of the present invention. In general, optical subassembly 10 includes an optical header 12 which is used to support an active optical device and provide electrical connections to the device. In accordance with the teachings of the present invention, an aligned optical substrate configuration 14 is disposed within a depression 16 formed in header 12. As will be described in detail below in association with FIGS. 2-4, aligned substrate configuration 14 is formed to both support an active optical device 18 and provide alignment between active device 18 and a coupling lens 20. An optical fiber receptacle 22 is illustrated as attached to header 12, with a fiber ferrule 24 inserted within an axial bore 26 of receptacle 22. An optical fiber 28 is included, as shown, within ferrule 24. 
     As mentioned above, an advantage of the design of the present invention as illustrated in FIG. 1 is the ability to provide completely passive optical alignment between active device 18, coupling lens 20, and optical fiber 28. To analyze this alignment, it is helpful to discuss the actual assembly process used to attach the various components to each other: (1) Initially, as will be discussed in detail below, submount piece part 32 for supporting active device 18 is positioned with depression 16 formed in header 12. The location of depression 16 may be controlled, with respect to the outer periphery 30 of header 12, within a tolerance of approximately ±2 mil. Active device 18 is then positioned on a solder preform formed on submount 32 (as will be discussed in detail below) and the solder is reflowed to provide alignment of active device 18 with respect to alignment fiducials on piece part 32 (see FIG. 5). (2) The next step in the process is to attach submount piece part 38 (including coupling lens 20) to piece part 32 so as to form aligned submount configuration 14. As will be discussed in detail below, the ability to provide alignment fiducials on piece parts 32 and 38 results in alignment of active device 18 to coupling lens 20 within a tolerance of ±2 μm. (3) Receptacle 22 (including ferrule 24 within bore 26) is next attached to header 12, as shown in FIG. 1, so as to be in physical contact with the outer periphery 30 of header 12. The fabrication process utilized to form receptacle 22 allows for bore 26 to be located within the center of receptacle 22 within a tolerance of ±0.2 mil. The actual attachment of receptacle 22 to header 12 provides initial alignment of approximately 2-3 mil. (4) The process of attaching receptacle 22 to header 12 results in the capturing of lens 20 within axial bore 26, (with, preferably, a predetermined minimal clearance between piece parts) where the alignment of lens 20 with bore 26 will be within the range of ±0.2-0.3 mil. As a result of these process steps, therefore, optical fiber 28 (located within ferrule 24 inside bore 26) will be aligned to the previously aligned combination of coupling lens 20 and active device 18 (as described in step (1)), the fiber to lens alignment being within the range of ±0.2-0.3 mil. Thus, the passive attachment scheme of the present invention results in an optical subassembly with alignment between components well within acceptable values. 
     The mechanical design of the inventive optical subassembly is also important to the reliability of the arrangement. In particular, as seen by reference to FIG. 1, receptacle 22 may be formed so as to avoid physical contact with aligned optical substrate configuration 14. Therefore, any unexpected movement of receptacle 22 will not effect the optical alignment between active device 18 and lens 20. Further, any mechanical force exerted by ferrule 24 onto receptacle 22 is transferred to header 12, and will not disturb the optical alignments described above. As seen, therefore, the mechanical design of the present optical subassembly which decouples the optical alignments from the mechanical mating of the receptacle to the header is critical, since the critical alignment is between device 18 and lens 20 (controlled within ±5 μm), with the alignment between optical fiber 28 and configuration 14 being more forgiving (±0.5 mil). Thus, when mated, the various piece parts provide optical alignment between the active device and the optical fiber. 
     FIG. 2 illustrates, in a detailed view, an exemplary aligned optical substrate configuration 14 formed in accordance with the teachings of the present invention. As shown, subassembly 14 includes a first substrate member 32 which is utilized as a mount for active device 18 (for example, an LED, photodiode or laser). The position of active device 18 with respect to first member 32 may be controlled by a solder self-alignment technique, as discussed in detail below in association with FIG. 5. Referring again to FIG. 2, a plurality of alignment fiducials 34 are formed at predetermined locations on top surface 36 of substrate 32. For example, alignment fiducials 34 may comprise pyramidal detents formed in surface 36 of first substrate 32. Substrate 32 may comprise any suitable material, including but not limited to, silicon. A second substrate member 38 is utilized as a holder for lens 20, as shown in FIG. 2. Further, second substrate 38 includes a plurality of alignment fiducials 40 which will mate with alignment fiducials 34 of first substrate 32 upon attachment of the first and second members. As shown in FIG. 2, second substrate 38 is processed to include alignment fiducials 42 on a first surface 44, while capturing lens 20 in a larger aperture 46 formed within the opposing surface 48. An additional opening 50 is formed in first surface 44 to accommodate active device 18 and provide an unobstructed optical signal path between active device 18 and lens 20. For the particular embodiment illustrated in FIG. 2, alignment fiducials 34 and 42 are formed as pyramidal detents, with spherical members 52 disposed therebetween. When first and second substrates 32, 38 are joined, spherical members 52 are in physical contact with both sets of pyramidal detents 34,42, providing mechanical joining of first substrate 32 to second substrate 38, as well as optical alignment between active device 18 and lens 20. In this particular configuration, surfaces 36 and 42 are in physical contact after mating. The dimensions of this configuration are such that when used with an optical transmitting device, a collimated beam will exit lens 20 (when used with an optical receiving device, therefore, a collimated incoming beam will be focused on the active region of device 18). 
     Depending upon the numerical aperture of the active device and the size of the lens, various parameters of aligned optical substrate configuration 14 may be controlled to provide optimal coupling efficiency between device 18 and lens 20. FIG. 3 illustrates an alternative aligned optical substrate configuration 140 wherein a larger optical alignment sphere 520 is utilized to provide separation between first substrate 32 and second substrate 38, which may be necessary with certain active devices 18 to maximize coupling efficiency. The dimensions of the various detents and openings in substrates 32,38 are essentially identical to those of configuration 14 of FIG. 2. Therefore, an advantage of the arrangement of the present invention is that coupling efficiency may easily be achieved merely by careful choice of the spheres used to control the separation between substrates 32,38. The only constraint on sphere dimension being, obviously, that the spheres must remain in physical contact with alignment fiducials 34,40. As an alternative, the size of coupling lens 20 may also be varied to maximize coupling efficiency. FIG. 4 illustrates an exemplary aligned optical substrate configuration 1400 utilizing a coupling lens 200 with a smaller diameter d than the diameter D of coupling lens 20 (as illustrated in FIG. 3). As shown in FIG. 4, opening 460 in second substrate 380 may be modified so as to physically capture smaller lens 200. It is to be understood that any desired combination of substrate separate, lens size, or even substrate size may be used to provide maximum coupling efficiency between active device 18 and lens 20 in accordance with the teachings of the present invention. 
     A necessary element in providing optical alignment between active device 18 and lens 20, regardless of the dimensions of configuration 14, is the ability to accurately position active device 18 with respect to alignment fiducials 34. FIG. 5 contains a top view perspective of first substrate 32, indicating the position of an exemplary arrangement of three such alignment fiducials 34 with the position of electrical bond pads 60 and a solder preform 62 (or other suitable disposed/deposited solder), where preform 62 is utilized to provide physical attachment and electrical coupling to active device 18 (not shown). Various processes in silicon technology exist such that bond pads 60 may be disposed within ±1 μm tolerance with respect to etched alignment fiducials 34. Active device 18 (not shown) may be located on solder preform 62 and subsequently first substrate 32 is heated to provide physical attachment of active device 18 to electrical bond pads 60. As is known in solder reflow processing, the heating will result in liquifying solder preform 62 such that the surface tension will provide self-alignment of active device 18 to underlying electrical bond pad 60. Therefore, as long as electrical bond pad 60 is formed within a predetermined tolerance with respect to alignment fiducials 34, active device 18 will also be aligned. Although a plurality of three such alignment fiducials is illustrated in FIG. 5, is it to be understood that any desired number of such alignment fiducials may be utilized in association with the teachings of the present invention. 
     FIG. 6 illustrates, in an exploded view, a complete assembly utilizing the passive aligned subassembly of the present invention. As shown, first substrate 32 is positioned within depression 16 of header 12, where electrical leads 70 and connections 72 are clearly illustrated in this view. In particular, connections 72 are attached by wirebonds 74 to desired electrical bond pads 60 formed on first substrate 32 (bond pads 60 being visible in the view of FIG. 5). Second substrate 38, including lens 20, is illustrated as positioned between first substrate 32 and receptacle 22. Not obvious from this view, but depicted clearly in FIG. 1, is the provision for receptacle 22 to mate with the outer periphery 30 of header 12 and the subsequent mating of axial bore 26 of receptacle 22 with lens 20, to provide for alignment between fiber 28 and aligned optical substrate configuration 14. Thus, by virtue of the passive alignment arrangement of the present invention, active device 18, coupling lens 20 and fiber 28 will be aligned as the various piece parts are brought together, with no need for any active alignment processes to be used. 
     It is to be understood that various materials may be used to form the substrate members discussed above in accordance with the teachings of the present invention. Further, other fiducial features (for example, interlocking grooves and ridges) may be used in place of the pyramidal detents discussed above. Many other variations to these and other aspects of the present invention may be made by those skilled in the art and are considered to fall within the spirit and scope of the teachings of the present invention.