Abstract:
A method of forming a hybrid optical component includes the steps of masking and etching a pattern on a core layer of a planar optical component to define at least one alignment element and an optical element and subsequently overcladding the optical element such that a passive platform is formed which exposes a surface of the optical element such as a waveguide and an alignment element for receiving a mirror image alignment element formed on an active platform including an active device, such as a laser. Hybrid components include a passive platform having an alignment element formed therein and a waveguide for receiving an active platform with a mating mirror image alignment element and an active device which aligns with the waveguide when the platforms are mated. Such a fabrication method and resulting optical component provide a highly efficient, self-aligning passive and active component platforms which greatly reduce the cost of fabrication of hybrid optical circuits as well as improve their reliability and reduce their cost.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is application claims the benefit of European Patent Application No. 00400201.0 filed on Jan. 25, 2000, the content of which is relied upon and incorporated herein by reference in its entirety, and the benefit of priority is hereby claimed. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to planar optical devices and particularly to an alignment system for aligning active devices with planar, passive optical devices. 
     2. Technical Background 
     Optical networks used in the communications industry require many complex optical components, examples of which include NXM switches, gain-flattening filters, variable attenuators, and add-drop multiplexers. An efficient way to design such components is to use planar optical devices inasmuch as many optical functions can be implemented on the same substrate. Furthermore, active devices can be added to the planar platform in order to create hybrid components delivering many of the functions needed in optical networks in a relatively compact package used for switching, monitoring, wavelength multiplexing, demultiplexing, wavelength conversion, and the like. 
     One difficulty with manufacturing hybrid devices is the precise alignment of active devices such as semi-conductor lasers, photo diodes and the like with the passive devices, such as waveguides, couplers, and switches on the same substrate. In the past, active alignment has been employed where the optical performance of a hybrid component is monitored as the active device is physically moved into alignment until, for example, a semi-conductor laser is aligned with a passive silica waveguide. The output from the waveguide is monitored until its optimal output is detected and, at such time, the active device is affixed, such as by soldering, to the platform of the passive device. This method is extremely time consuming and requires a complex experimental set-up to perform such alignment. 
     Another method employed in the prior art is the use of alignment marks on, for example, an optical waveguide substrate and separate alignment marks on an active platform and the subsequent alignment of the respective alignment marks to one another when the active device platform is joined with the passive device platform. This too requires precise placement of alignment marks initially on both platforms and the subsequent precise alignment of the active and passive platforms during their joining and bonding. 
     There exists a need, therefore, for a system and resultant hybrid component in which an active device can be precisely aligned to a planar optical component of a passive platform such that the active device is precisely coupled to the component. As hybrid optical components become increasingly in demand and the number of optical functions implemented on a single wafer increases, so too does the need to realize efficient and precise alignment of passive and active optical devices. 
     SUMMARY OF THE INVENTION 
     The method and resultant structure of the present invention provides such precise alignment of hybrid components by forming alignment standoff elements on a passive platform which matingly receive the mirror image structure of an active platform containing, for example, an active element to be aligned a waveguide of the passive platform. The two platforms precisely self-align with one another without the need for experimental positioning of the active device with respect to the passive device or the utilization of indexing marks in an effort to align the passive and active platforms. The method of forming a hybrid device according to the present invention includes the steps of masking and etching on a core layer of a planar optical component to define at least one standoff alignment element and an optical element and subsequently overcladding the optical element such that a passive platform is formed which exposes an optical element such as a waveguide and standoff alignment element for receiving a mirror image alignment element of an active platform including an active device, such as a solid state laser. 
     Hybrid components embodying the present invention include a passive platform having at least one standoff alignment element and an optical component formed therein for receiving an active platform with a mating mirror image alignment element. Such a fabrication method and resulting optical component provide a highly efficient, self-aligning passive and active component platforms which greatly reduce the cost of fabrication of hybrid optical components as well as improve their reliability and reduce their cost. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings. 
     It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of one step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 2 is a schematic view of an additional step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 3 is a schematic view of an additional step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 4 is a schematic view of an additional step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 5 is a schematic view of an additional step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 6 is a schematic view of an additional step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 7 is a schematic view of an additional step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 8 is a schematic view of an additional step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 9 is a schematic view of an additional step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 10 is a schematic view of an additional step in the process of manufacturing a hybrid optical component according to the present invention; 
     FIG. 10 a  is a cross-sectional view through section line Xa—Xa of FIG. 10; 
     FIG. 11 is a schematic view of the resultant passive platform manufactured according to the method of the present invention; 
     FIG. 12 is a schematic view of an active platform, including an active device which mates with the passive platform shown in FIG. 12; and 
     FIG. 13 is a schematic view of a hybrid optical component manufactured according to the present invention and including the active platform of FIG. 12 joined to the passive platform of FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, there is shown an initial preform of a planar optical circuit which in its entirety may include several optical components, such as switches, couplers, or multiplexers, and which includes both planar passive optical components and active devices, such as photo diodes, solid state lasers, and the like. For the purpose of clarity, however, the illustrations of the figures are of a single passive component and a single active component which form a part of an overall optical circuit. In FIG. 1, a section of a wafer for this optical circuit is shown and includes a planar layer of silica or silicon  10  forming a substrate, an under-clad layer  12  which is a silica (S i O 2 ) and dopant, such as boron, germanium, or the like, resulting in an index of refraction at 1550 nm of from about 1.44 to about 1.55. Deposited in a conventional manner over the under-clad layer  12  is a core layer  14  of silica having a different but conventional dopant mixture providing an index of refraction of approximately 1% greater than that of the under-clad layer  12  and, for example, of from about 1.5 to about 1.6. The structure shown in FIG. 1 is conventionally formed by typical processes, such as flame hydrolysis deposition, plasma-enhanced chemical vapor deposition (PECVD) or the like of the under-cladding on the substrate and the core layer on the under-cladding. 
     The next step in the process is illustrated in FIG.  2  and involves the depositing by sputtering or evaporation of a metallic layer  16  onto the exposed surface of the core layer  14 . The metallic layer may be any suitable conductive or semi-conductive material, such as aluminum, chromium, silicon, nickel or the like, having a deposition thickness of from about 0.2 to about 5 μm. 
     Subsequently, as shown in FIG. 3, a pattern of photo-resist material  15  is deposited on metallic surface  16  utilizing a mask to define what ultimately will become standoff alignment elements  18  and  22 , each including a pair of spaced-apart legs  17  and  19 , and  21  and  23 , respectively. As will be described in greater detail below, the patterned photo-resist layer  15  defines the shape and location of standoff alignment elements and a passive optical element  20 , such as a waveguide, and their respective position. Thus, the standoff elements  18  and  22  will become precisely positioned in fixed relationship and in alignment with waveguide  20  during the printing of the patterned photo-resist layer  15  onto the metallic layer  16  of the structure shown in FIG.  3  and by the subsequent processing steps. 
     The next step is illustrated in FIG.  4  and is the etching away of the exposed metallic layer  16  shown in FIG. 3 to expose the core material  14  with the metallic layer  16  underlying the photo-resist  15  defining elements  18 ,  20  and  22  remaining. Next, the photo-resist layer is removed as shown in FIG. 5 by the chemical washing of the photo-resist exposing the metallic mask corresponding to elements  18 ,  20  and  22  having the same pattern and relative alignment of the original photo-resist pattern only comprising the remaining exposed pattern of conductive material  16 . As seen in FIG. 6, the core material  14  is then etched away, such as by reactive ion etching, resulting in the three-dimensional structure shown in FIG. 6 with the core material  14  remaining under the metallic surfaces  16  of standoff alignment element  18 , waveguide  20  and standoff alignment element  22 . The under-cladding layer  12  is now exposed in the areas surrounding waveguide  20  and elements  18  and  22 . The under-cladding layer  12  can be partially etched. 
     Next, the upper surface of the parallel rectangular and spaced-apart legs  17  and  19  of standoff element  18  and legs  21  and  23  of standoff element  22  are coated with a photo-resist material  25  to protect the metallic surfaces thereon during the next step, shown in FIG. 8, in which the metallic surfaces  16  of the waveguide  20  and the waveguide overlap sections  26  and  27  of standoff elements  18  and  22 , respectively, are etched away by conventional wet or dry etching. The photo-resist material  25  is then washed away such that the metallic surface  16  on legs  17 ,  19 ,  21  and  23  of standoff alignment elements  18  and  22 , respectively, is exposed. 
     An overclad layer  30  is next deposited by, for example, flame hydrolysis deposition over the structure of FIG. 8, as illustrated in FIG.  9 . The overcladding  30  is silica with a dopant providing a similar index of refraction to that of the under-clad layer  12  but having a lower melting point, such as approximately 1240° C. as opposed to the melting point of the under-clad layer and substrates  10  and  12  of, for example, 1340° C. Thus, the flame hydrolysis deposition of the overcladding  30  does not affect the geometry of the substrate under-cladding, waveguide or standoff alignment elements previously formed and illustrated in FIG.  8 . Subsequent to the overcladding, the top surface of the overclad layer  30  is coated with aluminum, chromium, or a silicon metallic layer  32  (FIG. 9) by conventional sputtering or evaporation techniques to cover the entire top surface of the overclad layer  30 . 
     Next, a rectangular pattern of photo-resist  34  is applied over the metallic layer  32  only at the rear of the structure shown in FIG.  9 . Waveguide element  20  and overlap sections  26  and  27  are partially covered. The exposed metal surface  32 , above waveguide element  20  and standoff elements  18  and  22 , is then removed by wet or dry etching exposing the overclad layer  30 , as seen in FIG.  10  and cross section FIG. 10 a . The photo-resist  34  is subsequently washed away, exposing the metallic layer  32  previously protected by photo-resist layer  34 . At the end of this process, the metallic layer  32  over a part of the elements  20 ,  26 , and  27  is now exposed, and the overcladding  30  at the forward section of the structure over the standoff alignment elements  18  and  22  is exposed. The photo-resist masking  34  defines a precise boundary between the front and rear sections of the resulting structure which results, as described below, in a vertical wall  38  (FIG.  11 ). One key step of the process is to leave the mask in place during the core etching above the alignment elements before the overclad deposition. This embedded layer acts as a stop etch layer during the partial overclad etching. 
     As illustrated in FIG. 11, the uncovered overcladding  30  at the front of the passive platform  40  so formed is etched away, again using a conventional etching process such as reactive ion etching, to once again expose the parallel longitudinally extending legs  17  and  19  of standoff alignment element  18  and parallel longitudinally extending legs  21  and  23  of standoff element  22 . The embedded layer  16  acts as a stop etch layer to protect standoff elements  17 ,  19 ,  21 , and  23 . The etching may extend somewhat deeper into the under-clad layer  12  to form channels  35  and  37  between standoff elements  17  and  19  and  21  and  23 , respectively, and a wide channel  36  between legs  19  and  21 . The reference surface for Z-axis alignment of the active device described below is the upper metallic exposed surface  16  of the standoff elements  18  and  22 . The vertical end wall  38  of the remaining overcladding layer  30  is precisely formed during the etching to, as described below, provide Y-axis alignment of the active platform on the passive platform  40 . The end surface  28  of waveguide  20  formed of the core material  14  may be coated with a titanium dioxide (T i O 2 ) or other materials to reduce reflection when an active device, such as a solid state laser, is positioned on the passive platform as now described in connection with FIGS. 12 and 13. 
     An active platform  50  is shown in FIG. 12 which has alignment elements which are a mirror image of standoff alignment elements  18  and  22  of passive platform  40 . Active platform  50  is made of compounds of amorphous materials of group III-V elements from the periodic table, for example, I n P and an active layer of I n G a A s P to form an embedded laser waveguide  54  which is centered therein and precisely aligned with alignment elements  58  and  62  comprising longitudinally extending, rectangular, downwardly projecting ridges which matingly fit within the channels  35  and  37  of the passive platform  40 . The active platform  50  aligns with the passive platform  40  such that the active end  55  of laser waveguide  54  aligns and is centered with end  28  of waveguide  20 , as seen in FIG. 13, when the active platform  50  is positioned on the passive platform  40  and bonded thereto by, for example, thermal-compression (in one embodiment) or by any other suitable means, such as conventional bonding agents. The active platform is made in such a way to provide mirror image inter-fitting alignment elements  58  and  62  which fit within channels  35  and  37  of platform  40  and channels  70 ,  72 ,  74 , and  76  which receive legs  17 ,  19 ,  21  and  23 , respectively, of the standoff alignment elements  18  and  22  of platform  40 . The alignment elements of the active device are a mirror image of the alignment elements of the passive platform and are directly etched on the top surface of the active device. 
     The rear wall  52  of the active platform  50  abuts against and aligns with the front wall  38  of active platform  40  to provide Y-axis alignment as shown by arrow Y in FIG. 13, while the inter-fitting alignment elements and channels provide X-axis alignment as shown by arrow X in FIG.  13 . The Z-axis alignment is achieved by the metallic surfaces  16  on top of standoff elements  18  and  22  fitting against the lower surfaces  51  of each of the channels  70 ,  72 ,  74 , and  76 , which are precisely etched to provide substantially centered alignment of the active laser waveguide  54  with waveguide  20 . The mirror image alignment elements formed on the active platform  50  are formed by reactive ion beam etching or the like utilizing mirror image masking to that employed in the manufacturing of the passive platform  40 . 
     The hybrid component  60 , shown in FIG. 13, is illustrative only of the process and shows a hybrid component which includes a solid state laser  54  coupled to and aligned with waveguide  20 . An optical circuit will include numerous other optical components which may be integrally formed at the same time as the waveguide  20  is formed and during the same processing steps. Since the positive alignment elements of the passive platform are self-aligned with the negative elements of the active platform and vice versa, X-axis alignment accuracy is provided by the precision of the formation positive/negative alignment elements, which is well within 0.5 μm. The Y-axis precision likewise is defined by the precision of alignment of the end walls  38  and  54  of the passive and active platforms, respectively, and also is within 0.5 μm. The Z-axis alignment precision is controlled by the etching depth of the surfaces of the active device and can readily be controlled within 0.1 μm utilizing standard reactive ion etching or reactive ion beam etching processes. Although the alignment elements disclosed in the preferred embodiment are parallel, generally rectangular extending projections and channels with integral end walls, such as walls  38  and  52 , it is understood that the shape, placement, and number of the alignment elements can be varied as long as they provide inter-fitting surfaces between the passive platform and the active platform in X, Y, and Z directions. Thus, alignment elements may take the form of square, rectangular, or triangular blocks and similarly shaped receiving sockets. 
     With the method of the present invention, complex hybrid optical components can be fabricated to provide precise alignment between passive and active elements utilizing a cost effective manufacturing method. It will become apparent to those skilled in the art that various modifications to the preferred embodiment of the invention as described herein can be made without departing from the spirit or scope of the invention as defined by the appended claims.