CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of United States provisional patent application number 63/171,849, filed April 7, 2021, by Rongsheng Miao et al, and titled “Fiber Bonding with Enhanced Thermo-Mechanical Strength,” which is incorporated by reference.

TECHNICAL FIELD

[0002] The present disclosure is generally related to fiber optical packaging or assemblies, and more specifically to providing fiber bonding with enhanced thermo-mechanical strength.

BACKGROUND

[0003] Increasing demand in the data communications sector is spurring further growth in photonics components. Photonic components enable the manipulation of light. Photonics components require accurate alignment of glass optical fibers and accurate alignment of optical components.

SUMMARY

[0004] A first aspect relates to a fiber optical assembly. The fiber optical assembly includes a fiber assembly unit (FAU). The FAU includes a fiber in between a top block and a bottom block. The fiber optical assembly includes an optical chip having a waveguide. A first bonding layer couples a first edge of the optical chip to the FAU and permits the waveguide to receive light from the fiber of the FAU. A bridge lid couples to the top block of the FAU. A second bonding layer couples the bridge lid to the optical chip.

[0005] In a first implementation form of the fiber optical assembly according to the first aspect, the fiber optical assembly also includes a lid mounted to the optical chip. The second bonding layer bonds the lid to the bridge lid.

[0006] In a second implementation form of the first aspect as such or any preceding implementation form of the first aspect, the fiber optical assembly also includes a thermal stress reduction zone.

[0007] In a third implementation form of the first aspect as such or any preceding implementation form of the first aspect, the thermal stress reduction zone comprises a chamfer in the top block of the FAU.

[0008] In a fourth implementation form of the first aspect as such or any preceding implementation form of the first aspect, the thermal stress reduction zone comprises a lid recessed from an edge of the optical chip.

[0009] In a fifth implementation form of the first aspect as such or any preceding implementation form of the first aspect, the lid mounted to the optical chip is recessed between 03 0.5 millimeters (mm) from the edge of the optical chip.

[0010] In a sixth implementation form of the first aspect as such or any preceding implementation form of the first aspect, the optical chip is flip-chip bonded to the FAU.

[0011] In a seventh implementation form of the first aspect as such or any preceding implementation form of the first aspect, the fiber optical assembly also includes a solder grid ball array coupled to the optical chip to enable an electrical connection to a sub-chip. [0012] In an eighth implementation form of the first aspect as such or any preceding implementation form of the first aspect, the fiber optical assembly also includes a mode converter. The bridge lid is coupled to the top block of the FAU and the mode converter.

[0013] In a ninth implementation form of the first aspect as such or any preceding implementation form of the first aspect, the first bonding layer extends in a vertical direction relative to the fiber and the second bonding layer extends in a horizontal direction relative to the fiber.

[0014] In a tenth implementation form of the first aspect as such or any preceding implementation form of the first aspect, a viscosity of a bonding epoxy of the second bonding layer is over 100,000 centipoise (cPs).

[0015] In an eleventh implementation form of the first aspect as such or any preceding implementation form of the first aspect, a curing shrinkage of the bonding epoxy of the second bonding layer is less than one-percent.

[0016] In a twelfth implementation form of the first aspect as such or any preceding implementation form of the first aspect, a glass transition temperature (T _g ) of the bonding epoxy of the second bonding layer is greater than 100 degrees Celsius (C).

[0017] In a thirteenth implementation form of the first aspect as such or any preceding implementation form of the first aspect, the bridge lid is formed of a single block of glass.

[0018] In a fourteenth implementation form of the first aspect as such or any preceding implementation form of the first aspect, the bridge lid is bonded to the top block of the FAU using a thin layer of epoxy approximately 10 micrometers thick.

[0019] A second aspect relates to a fiber optical assembly. The fiber optical assembly includes a fiber assembly unit (FAU). The FAU includes a fiber in between a top block and a bottom block. The fiber optical assembly includes an optical chip having a waveguide. A first bonding layer couples a first edge of the optical chip to the FAU and permits the waveguide to receive light from the fiber of the FAU. The fiber optical assembly includes a thermal stress reduction zone comprising a chamfer in the top block of the FAU. A bridge lid couples to the top block of the FAU. A second bonding layer couples the bridge lid to the optical chip. A solder grid ball array is coupled to the optical chip to enable an electrical connection to a sub-chip.

[0020] In a first implementation form of the fiber optical assembly according to the second aspect, the fiber optical assembly also includes a mode converter. The bridge lid is coupled to the top block of the FAU and the mode converter.

[0021] In a second implementation form of the second aspect as such or any preceding implementation form of the second aspect, the optical chip is flip-chip bonded to the FAU.

[0022] A third aspect relates to a method for assembling a fiber optical assembly. The method includes pre-bonding a bridge lid to a top block of a fiber assembly unit (FAU) to form a FAU- bridge lid subassembly. The FAU includes a fiber in between the top block and a bottom block bonding a lid to an optical chip. The method also includes aligning the fiber to a waveguide of the optical chip; bonding the FAU to the optical chip using a first bonding layer between the FAU and an edge of the waveguide of the optical chip; and bonding the bridge lid to the optical chip using a second bonding layer between the bridge lid and the optical chip.

[0023] In a first implementation form of the method for assembling a fiber optical assembly according to the third aspect, the first bonding layer extends vertically and wherein the second bonding layer extends horizontally.

[0024] For the purpose of clarity, any one of the foregoing implementation forms may be combined with any one or more of the other foregoing implementations to create a new embodiment within the scope of the present disclosure. These embodiments and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts.

[0026] FIG. 1 is schematic diagram illustrating a fiber optical assembly.

[0027] FIG. 2 is schematic diagram illustrating a fiber optical assembly with a mode converter.

[0028] FIG. 3 is schematic diagram illustrating a fiber optical assembly in accordance with an embodiment of the present disclosure.

[0029] FIG. 4 is schematic diagram illustrating a fiber optical assembly in accordance with an embodiment of the present disclosure.

[0030] FIG. 5 is schematic diagram illustrating a fiber optical assembly in accordance with an embodiment of the present disclosure.

[0031] FIG. 6 is schematic diagram illustrating a fiber optical assembly in accordance with an embodiment of the present disclosure.

[0032] FIG. 7 is schematic diagram illustrating a fiber optical assembly in accordance with an embodiment of the present disclosure.

[0033] FIG. 8 is schematic diagram illustrating a fiber optical assembly in accordance with an embodiment of the present disclosure. [0034] FIG. 9 is schematic diagram illustrating a fiber optical assembly in accordance with an embodiment of the present disclosure.

[0035] FIG. 10 is a flowchart illustrating an assembly sequence in accordance with an embodiment of the present disclosure

DETAILED DESCRIPTION

[0036] It should be understood at the outset that although an illustrative implementation of one or more embodiments are provided below, the disclosed systems and/or methods may be implemented using any number of techniques, whether currently known or in existence. The disclosure should in no way be limited to the illustrative implementations, drawings, and techniques illustrated below, including the exemplary designs and implementations illustrated and described herein, but may be modified within the scope of the appended claims along with their full scope of equivalents.

[0037] The present disclosure provides various embodiments of fiber optical assemblies or fiber optical packaging that substantially enhance the thermo-mechanical strength of the FAU- waveguide bonding. The disclosed embodiments increase the likelihood that a fiber optical assembly survives high temperature processes.

[0038] In fiber optical packaging, a fiber or fiber assembly unit (FAU) needs to be coupled and bonded to the edge of waveguide port of an optical chip to let light travel between the waveguide/optical chip and the fiber/FAU. The diameter of the optical core of the waveguide and the core of the fiber are typically a few microns. These dimensions make the design of fiber-to- chip interfaces challenging because any tiny shift in coupling and bonding results in a large coupling loss, which reduces efficiency and bandwidth. Further, significant thermal stress occurs during the epoxy bonding, curing, and baking processes, as well as the subsequent reliability tests. The thermal stress can cause a change or shift in the FAU-waveguide bonding. This change or shifting is a plastic change, meaning the change does not return to its original state even after the thermal stress is released. Therefore, the change leads to permanent loss of the FAU-waveguide coupling. As the packaging techniques evolve in the silicon photonics industry, especially in the highly integrated opto-electric packaging, some applications require the fiber-waveguide bonding to be even more thermally stable to pass the solder ball grid array (BGA) reflow process in which the reflow temperature is in the vicinity of 260 degrees Celsius (°C).

[0039] FIG. 1 is schematic diagram illustrating a fiber optical assembly 100. The fiber optical assembly 100 connects a FAU 113 to the edge of a waveguide (WG) 104 of a photonic or optical chip 102 using a bonding scheme used in the current opto-electronic industries. The edge or lateral face of the WG 104 may be referred to herein as a port or WG port. The FAU 113 includes a top block 110, a fiber 112, and a bottom block 114. The fiber 112 may be a single fiber-optic strand or may comprise multiple fiber-optic strands. The fiber 112 may be single-mode fibers (SMFs) or polarization-maintaining (PM) fibers. The fiber 112 is sandwiched between the top block 110 and the bottom block 114.

[0040] The FAU 113 is bonded to the optical chip 102 using a bonding layer 108. The optical chip 102 may include both electronic and photonic circuits that are used together to increase efficiency. Photonic circuits use photons instead of electrons. Photonic circuits are more efficient than electronic circuits because electronic circuits have to be charged and discharged to send a bit of data, whereas photonic circuits do not. However, guiding a photon from one element to another (i.e., from the FAU 113 to the optical chip 102) is not as simple as soldering the two together. Alignment tolerances are much more exacting for photonic elements. Whereas connecting a wire to a contact-pad on an electronic chip involves aligning the components to within a few tens of micrometers of the correct position, connecting an optical fiber to a photonic chip can require three orders of magnitude more precision. Thus, the bonding layer 108 is relatively thick (e.g., typically around 10 micrometers (pm)) to allow 3 or 6-axis adjustment of the FAU 113 to minimize coupling loss between the port of the WG 104 of the optical chip 102 and the fiber 112. The WG 104 is a physical structure that guides electromagnetic waves in the optical spectrum. In the depicted embodiment, the light beam is coupled in/out from the WG 104 from lateral sides, thus always propagating in the same plane. A lid 106 is used to reinforce the bonding strength. The lid 106 is bonded to the optical chip 102 using a thin layer of epoxy.

[0041] FIG. 2 is schematic diagram illustrating a fiber optical assembly 200. The fiber optical assembly 200 includes a mode converter 115 or multi-channel mode converter in addition to the same components of the fiber optical assembly 100 described in FIG. 1. The mode converter 115 is coupled to the FAU 113. The mode converter 115 uses a silicon (Si) lens 118 and a glass block 116 to convert the mode field diameters (MFD) between the WG 104 and the fiber 112 to minimize coupling loss. The Si lens 118 may be formed from two or more lenses. In particular, the Si lens 118 converts the mode diameter of the WG 104 to the same mode diameter as the fiber 112 and vice versa. The glass block 116 provides an optical path between the Si lens 118 and the FAU 113. The mode converterl 15 is bonded to the edge of the WG 104 of the optical chip 102 by the bonding layer 108 similar to the method used in the FIG. 1.

[0042] There are several weaknesses in the bonding schemed used in the fiber optical assembly 100 of FIG. 1 and the fiber optical assembly 200 of FIG. 2. First, there is only one bonding layer (i.e., bonding layer 108) to hold the FAU 113 to the WG 104 of the optical chip 102. Considering the physical size of the FAU 113 as well as the long fiber 112 attached, the bonding strength is weak at high temperature because the bonding epoxy tends to soften when temperature approaches to its glass transition temperature (Tg). Second, the lid 106 and the top block 110 are two separate blocks. Therefore, a broken surface exists between them. Third, there is a stress concentration zone where the corner area of the top block 110 of FIG. 1 abuts the bonding layer 108, which that generates considerable thermal stress at an elevated temperature. The bonding scheme in FIG. 1 and FIG. 2 typically passes reliability tests conducted under a testing temperature of 100 °C. However, the bonding scheme in FIG. 1 and FIG. 2 rarely survives processes requiring higher temperatures, such as the 260 °C BGA reflow process. This is because the bonding strength of the bonding scheme gets weaker as the temperature goes higher due to the design drawbacks mentioned above.

[0043] To address the above issues, the present disclosure describes various bonding schemes that substantially enhance the thermo-mechanical strength of the FAU-waveguide bonding. The disclosed embodiments increase the likelihood that a fiber optical assembly survives high temperature processes such as the BGA reflow process.

[0044] FIG. 3 is schematic diagram illustrating a fiber optical assembly 300 in accordance with an embodiment of the present disclosure. The fiber optical assembly 300 includes a FAU 113 comprising the fiber 112 sandwiched between the bottom block 114 and the top block 110. The bonding layer 108 connects the edge of the waveguide 104 of the optical chip 102 to the FAU 113. The bonding layer 108 is an index- matching optical epoxy or other similar adhesive, meaning the refractive index of the epoxy/adhesive approximately matches the refractive index of the WG 104. In various embodiments, the refractive index is between about 1.40 to about 1.50.

[0045] A bridge lid 120 connects the top block 110 of the FAU 113 to the lid 106 mounted on top of the optical chip 102 using a second bonding layer 122. The second bonding layer 122 extending in the horizontal direction and the bonding layer 108 extending in the vertical direction provide a more mechanically stable structure than the fiber optical assembly 100 in FIG. 1. The second bonding layer 122 does not have an index matching requirement because the second bonding layer 122 is not connected to the WG 104 of the optical chip 102. The bonding epoxy used to fill the gap of the bonding layer 122 preferably has high viscosity, low curing shrinkage, and high glass transition temperature (Tg). In various embodiments, the viscosity of the bonding epoxy of the bonding layer 122 is over 100,000 centipoise (cPs), the curing shrinkage is about 1 percent (%) or smaller, and the Tg is about 100 °C or higher.

[0046] Additionally, the new bridge lid 120 physically bonds both the optical chip 102 and the FAU 113 using a single solid lid (i.e., one solid piece) that substantially increases the mechanical bonding strength of these two mating parts. In an embodiment, the bridge lid 120 is formed of a single block of glass. In addition, the fiber optical assembly 300 includes a stress relief design to reduce the thermal stress generated at the stress concentration zone where the corner area of the top block 110 abuts the bonding layer 108. In the depicted embodiment, a chamfer 124 is created at the corner of the top block 110 of the FAU 113 to reduce the stress concentration zone. The chamfer 124 is a transitional edge between two faces or a sloping surface at an edge or corner. The chamfer 124 substantially reduces the thermal stress during the series bonding processes as well as during reliability tests, which significantly reduces the thermal shifting of the FAU bonding.

[0047] FIG. 4 is schematic diagram illustrating a fiber optical assembly 400 in accordance with an embodiment of the present disclosure. The fiber optical assembly 400 depicts the optical chip 102 in a flip-chip scenario where the optical chip 102 is flipped over from the orientation depicted in FIG. 3. This results in the WG 104 being located near the bottom end of the optical chip 102 as shown in FIG. 4. The FAU 113, which comprises the fiber 112 sandwiched between the bottom block 114 and the top block 110, is connected to the WG 104 of the optical chip 102 by the bonding layer 108.

[0048] In this embodiment, the bridge lid 120 is bonded to the top block 110 of the FAU 113 and directly to the optical chip 102 by the second bonding layer 122 (i.e., no lid 106). In an embodiment, the bridge lid 120 is made of glass and can be pre-bonded to the top block 110 of the FAU 113 using a minimum layer of epoxy, typically about 10 pm in thickness. The thickness of the top block 110 of the FAU 113 is engineered to ensure there is an adequate gap between the bridge lid 120 and the surface of the optical chip 102 to allow active adjustment of the FAU 113 to the WG 104. In various embodiments, the gap between the bridge lid 120 and the surface of the optical chip 102 is controlled to be about 20 to 40 pm in width.

[0049] The fiber optical assembly 400 also includes a chamfer 124 at the corner area of the top block 110 to reduce the thermal stress generated at the stress concentration zone during the series bonding processes as well as during reliability tests. The chamfer 124 prevents excessive bonding epoxy from overflowing to the corner and thus the thermal stress at the corner is significantly reduced during high temperature processes. In various embodiments, thermo-mechanical simulation reveals that the thermal stress at the comer is reduced to 20 percent or less compared to the thermal stress without the stress relief. Thus, decreasing the thermal shift between the FAU 113 and the optical chip 102. This results in substantially increasing the mechanical strength and decreasing the coupling loss of the fiber optical assembly 400.

[0050] FIG. 5 is schematic diagram illustrating a fiber optical assembly 500 in accordance with an embodiment of the present disclosure. The fiber optical assembly 500 depicts the optical chip 102 bonded upright (i.e., not flip-chip bonded like the fiber optical assembly 400 in FIG. 4) to the FAU 113 using the bonding layer 108. In this embodiment, instead of adding the chamfer 124 at the corner area of the top block 110 to reduce the thermal stress generated at the stress concentration zone, the lid 106 is recessed away from the edge of the optical chip 102. In an embodiment, the recess distance from the edge of the optical chip 102 is around 0.3 millimeters (mm) to around 0.5 mm. This creates a thermal stress relief zone. [0051] The bridge lid 120 connects the top block 110 of the FAU 113 to the lid 106 mounted on top of the optical chip 102 using the second bonding layer 122. The epoxy bonding thickness of the second bonding layer 122 to the lid 106 is kept as thin as possible to minimize the shift during the high temperature processes.

[0052] FIG. 6 is schematic diagram illustrating a fiber optical assembly 600 in accordance with an embodiment of the present disclosure. The fiber optical assembly 600 is similar to the fiber optical assembly 500 in FIG. 5, except the chamfer 124 is added to the top block 110 in addition to recessing the lid 106 from the edge of the optical chip 102. This creates even more room to accommodate the excessive epoxy in the bonding process to ensure there is no thermal stress occurring at the corner.

[0053] FIG. 7 is schematic diagram illustrating a fiber optical assembly 700 in accordance with an embodiment of the present disclosure. The fiber optical assembly 700 includes the optical chip 102, which is flip-chip bonded to the FAU 113 by the bonding layer 108 as similarly described in FIG. 4. The fiber optical assembly 700 includes the chamfer 124 to reduce the thermal stress generated at the corner of the top block 110. The bridge lid 120 couples the top block 110 to the optical chip 102 using the second bonding layer 122. In the depicted embodiment, the fiber optical assembly 700 includes a BGA solder ball array 126. In an embodiment, the BGA solder ball array 126 is pre-attached to the surface of the optical chip 102 and then flip-chip bonded to a base plate (the base plate is not shown in the figure). The base plate can be either a silicon interposer in a two and a half dimensional (2.5D) packaging or a substrate in a three-dimensional (3D) packaging. An interposer is an electrical interface located between two connections. The purpose of an interposer is to spread a connection to a wider pitch or to reroute a connection to a different connection. The BGA solder ball array 126 provides an electronic connection for connecting the optical chip 102 to a sub-chip or other electrical component (not shown in figure).

[0054] FIG. 8 is schematic diagram illustrating a fiber optical assembly 800 in accordance with an embodiment of the present disclosure. The fiber optical assembly 800 is the same as the fiber optical assembly 500 in FIG. 5, but with the addition of the BGA solder ball array 126.

[0055] FIG. 9 is schematic diagram illustrating a fiber optical assembly 900 in accordance with an embodiment of the present disclosure. The fiber optical assembly 900 illustrates the bonding scheme presented herein on a fiber optical assembly having a mode converter 115 as described in FIG. 2. As described above, the mode converter 115 converts the mode sizes between the WG 104 and the fiber 112. The mode converter 115 is coupled to the WG 104 by the bonding layer 108. As shown in FIG. 9, the bridge lid 120 can span across the top block 120 and the mode converter 115 (e.g., the Si lens 118 and the glass block 116). The bridge lid 120 is then coupled to the lid 106 mounted on top of the optical chip 102 using the second bonding layer 122.

[0056] FIG. 10 is a flowchart illustrating an assembly sequence 1000 in accordance with an embodiment of the present disclosure. The assembly sequence 1000 can be used to assemble any of the various fiber optical assemblies disclosed herein. The assembly sequence 1000 can be programmed to be performed by various automated machinery. The assembly sequence 1000 starts, at step 1002, with pre-bonding the bridge lid 120 to the top block 110 of the FAU to form a FAU-bridge lid subassembly. At step 1004 of the assembly sequence 1000, the optical chip 102 is prepared by assembling all electrical and optical components as well as bonding the lid 106 to the optical chip 102 if needed. At step 1006, the FAU-bridge lid subassembly is actively aligned to the optical chip 102/lid 106 subassembly so that the fiber 112 is aligned with the WG 104 of the optical chip 102. At step 1008, when peak coupling efficiency is reached, the FAU-bridge lid subassembly is bonded to the optical chip 102/lid 106 subassembly using the bonding layer 108 and the second bonding layer 122 as described herein. At step 1010, the epoxy of the fiber optical assembly is ultraviolet (UV) cured and oven baked for a predetermined time.

[0057] In sum, these disclosed embodiments provide several technical improvements to significantly enhance the mechanical and thermo-mechanical strength of the FAU bonding to the edge of the waveguide. For instance, the two bonding layers that bond the FAU to the optical chip, a first vertical bonding layer and a second horizontal bonding layer (in the direction to the fiber), significantly enhance the mechanical strength of the fiber optical assembly. Additionally, thermo mechanical simulation indicates that the addition of the thermal stress reduction designs (e.g., using the chamfer 124) disclosed herein reduces the thermal shift between the FAU and the optical chip by about 50%, which significantly improves the thermo-mechanical strength as well as the coupling loss. Therefore, the disclosed embodiments can be used in the processes with higher temperature than the existing processes, such as, but not limited to, the 260C BGA reflow process, but the existing bonding scheme cannot handle.

[0058] The disclosed embodiments can be widely used in various opto-electronic packaging, especially in the BGA related packaging, including 2.5D and 3D packaging.

[0059] While several embodiments have been provided in the present disclosure, it should be understood that the disclosed systems and methods might be embodied in many other specific forms without departing from the spirit or scope of the present disclosure. The present examples are to be considered as illustrative and not restrictive, and the intention is not to be limited to the details given herein. For example, the various elements or components may be combined or integrated in another system or certain features may be omitted, or not implemented. [0060] In addition, techniques, systems, subsystems, and methods described and illustrated in the various embodiments as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable by one skilled in the art and could be made without departing from the spirit and scope disclosed herein.