Automated, Self-Aligned Assembly of 12 Fibers per Nanophotonic Chip with Standard Microelectronics Assembly Tooling

Tymon Barwicz1*, Nicolas Boyer2, Stephane Harel2, Ted W. Lichoulas3, Eddie L. Kimbrell3, Alexander Janta-Polczynski2, Swetha Kamlapurkar1, Sebastian Engelmann1, Yurii A. Vlasov1 and Paul Fortier2

1IBM T.J. Watson Research Center, 1101 Kitchawan Rd., Yorktown Heights, NY 10598 USA 2IBM Bromont, 23 Boul. de l’Aeroport, Bromont, QC J2L 1A3 Canada

3AFL Telecommunications, 170 Ridgeview Circle, Ducan, SC 29334 USA *tymon@us.ibm.com

Abstract Silicon photonics technology aims to leverage

microelectronic chip fabrication facilities to bring disruptive advancements in photonic circuits cost and complexity. However, the large scale deployment of silicon photonics is muted by the difficulty of cost-efficient and scalable, single-mode optical inputs and outputs. To disruptively improve on cost and scalability, we believe that the best approach is to enable existing high-throughput microelectronic packaging tools for single-mode photonic packaging. In this paper, we experimentally demonstrate such approach with automated assembly of standard-fiber arrays to photonic chips. We identify the main challenges and solutions to enabling high-throughput pick-and-place tooling for single-mode photonic assembly. These include challenges with fiber handling, placement accuracy and limitations in movement complexity. We present a manufacturability assessment of the employed fiber-to-chip self-alignment. We show through Monte Carlo tolerance analysis an expected manufacturing re-alignment accuracy of <1.3 um despite initial misalignments of up to ~40 um. We believe the approach proposed and demonstrated here can substantially improve on single-mode optical input and output cost and scalability.

Introduction Single-mode optical inputs and outputs (I/Os) to photonic

chips involve high-accuracy alignment of optical glass fibers to embedded waveguide couplers. Currently, this operation is either manual or partially automated with specialized tooling. This results in low throughput, low scalability and much higher cost than electrical I/Os. In fact, the cost of optical I/Os can exceed the chip cost by an order of magnitude or more and negate gains in chip-level cost and integration complexity attained with planar processing of integrated photonic structures. The negative cost impact of packaging is nowhere larger than in silicon photonics, which is leveraging decades of microelectronic processing to bring the chip itself to a new level of cost efficiency and photonic integration complexity [1, 2]. To enable the large scale deployment of silicon photonics, disruptive improvements in the following areas of optical I/Os are required:

optical connection cost,

scalability in optical port count per chip, and

scalability in manufacturing volume.

We argue that the best approach to achieving these goals is to enable standard microelectronic packaging equipment for optical I/Os. This direction is analogous to photonic chip fabrication already leveraging standard microelectronics wafer fabrication equipment. Significant gains in cost and throughput

can be achieved if existing infrastructure is applied. Common high-throughput microelectronic pick-and-place tools can perform up to 10,000 assemblies per hour.

The main challenges in applying standard, high-throughput microelectronic packaging equipment to optical I/Os are fiber handling and alignment accuracy. Fibers are generally handled with specialized tools and are not well suited for the vacuum pick-tips used in microelectronic equipment. In addition, low-loss single-mode connections require 1-2 um alignment accuracy, which is much tighter than the typical 10 um placement accuracy of high-throughput microelectronic pick-and-place equipment. To achieve low coupling loss, active optical fiber alignment is often used and involves optimizing the fiber position with real-time measurements of the coupling efficiency through electrical or optical probing of the chip the

Figure 1. Schematic of a fiber stub with an integrated

polymer lid. The exploded diagram is shown in (a) and the assembled structure in (b). The lid enables vacuum pick tip handling of the fiber ribbon and maintains the fiber pitch

within the re-alignment range of v-grooves on chip.

fiber is being aligned to. Such approach exacerbates cost and throughput and is not compatible with high-throughput microelectronic equipment. Another approach is required to leverage microelectronic packaging facilities.

In this paper, we overcome these challenges to demonstrate automated and simultaneous assembly of 12 fibers per chip with three-dimensional self-alignment to single-mode photonics accuracy. Our approach is compatible with standard microelectronic high-throughput pick-and-place tools which is expected to provide significant gains in photonic packaging cost and scalability. Our current process could be applied to any number of fibers per chip as long as they are distributed in a one-dimensional array. We chose to work with 12 fibers due to the high availability of 12-fiber connectors and ribbons. We decided against two-dimensional fiber arrays despite their conceptual connection density advantage. Mainly, two-dimensional fiber arrays require vertical grating couplers, whose optical bandwidth is inherently small [3] and not compatible with the coarse wavelength division multiplexing (WDM) standards that are increasingly popular in high-growth applications. In fact, the use of WDM enables, in principle, bandwidths of multiple terabits/s per fiber and reduces considerably the number of fibers required in a given link. This suggests that the one-dimensional aspect of the array, which can still deliver tens of fibers to a chip, is not a practical limitation for the foreseeable future.

In addition to parallelized fiber assembly, we have previously proposed and reported a low-cost compliant polymer interface between optical fibers and nanophotonic waveguides [4-6]. The two approaches have different sets of risks and benefits. The selection of one approach over another depends on the specifics of a given application. The advantage of direct fiber-to-chip assembly, as described here, resides in the wide-band optical transparency of optical fibers while the

advantage of a compliant polymer interface resides in mechanical reliability. Direct connections of rigid fibers to a chip suffer from chip-package-interaction and other thermo-mechanical concerns.

Parallelized fiber assembly in microelectronic tools. Enabling parallelized single-mode fiber assembly to

photonic chips in standard high-throughput pick-and-place tools requires the resolution of a number of challenges, which can be summarized in three points:

1. pick-and-place handling of optical fibers, 2. lateral alignment of fibers to waveguide couplers on

chip, and 3. longitudinal butting of fibers to waveguide couplers on

chip. The solution to the first challenge is presented in Fig. 1. A UV-transparent polymer lid is assembled to the stripped-fiber end of a fiber stub. The fiber stub includes a short MT ferrule, which allows for a standard fiber connector interface at one end. The other end of the stub is formed of stripped and cleaved standard single-mode fibers that will interface with the waveguide couplers on chip. The lid enables the handling of the fiber stub by the bare fiber end with a vacuum pick tip. The importance of handling the stub by the fiber end and not by the ferrule, as well as the importance of lid UV transparency, will become obvious below.

The solution to the second challenge is presented in Fig. 2 and Fig. 3. Each fiber of the ribbon needs to be aligned with 1-2 um transversal accuracy to each waveguide coupler on chip. This is accomplished with a built-in V-groove array on chip. The fiber stub is picked by the polymer lid with a UV transparent pick-tip, positioned on top of the V-groove, and pressed in. For proper seating and re-alignment of the fibers in the grooves, the pick tip must exert a direct vertical force and thus be positioned straight on top of the fiber end of the stub. Holding the stub by the ferrule would not allow proper seating

Figure 2. Exploded schematic of a fiber stub to a

nanophotonic die assembly. An array of V-grooves integrated on the photonic chip provides lateral self-alignment of individual fibers to their corresponding

waveguide couplers. nanophotonic die.

Figure 3. Schematic of a fiber stub assembled to a nanophotonic die. The short MT ferrule provides a

standard fiber array optical interface. Electrical interfacing to the chip is done with standard flip-chip C4

connections to a laminate, which must be design with proper clearance for the fibers and lid.

of fibers in the V-groove array as a large torque component of the assembly force would lift the fiber ends. UV-cured optical adhesive is dispensed in the V-grooves prior to fiber assembly. Once the fibers are well seated in the V-grooves, the adhesive is sufficiently UV-cured through the vacuum picktip and lid to at least tack the fibers in place. The cure can be finalized outside the pick-and-place tool if needed.

The re-alignment capability of V-grooves is quite large and is calculated below to exceed +/- 40 um. After assembly, the accuracy of lateral alignment of the fiber core to the waveguide coupler on chip is limited by the V-groove and optical fiber fabrication accuracy. A tolerance analysis is presented below and shows 1.3 um final accuracy.

The third challenge, longitudinal fiber butting on waveguide couplers, is conceptually straightforward but is the hardest to implement in standard high-throughput pick-and-place tools. It is generally accomplished by sliding the fibers forward once they are seated in the V-groove. The issue here is that standard pick-and-place tools allow precise pressure controlled movements but only in the vertical direction and only after horizontal positioning. Hence, we can position the fibers in the V-grooves but not move them further in the plane of the chip for butting on waveguide couplers. Here, a pressure controlled horizontal displacement is needed after initial horizontal and vertical positioning. Our solution is shown in Fig. 4. We designed an angled sliding base under the chip that trigonometrically transfers a portion of the vertical placing

force into a horizontal butting force. The fibers are purposefully positioned in the V-grooves at a safe distance from the waveguides. The safe distance is established based on parts tolerances and placement accuracy. The distance shown in Fig.4 (c)-(d) has been significantly amplified for graphical clarity. After the fibers enter the V-grooves, the fiber stub will continue its vertical motion while inducing a sliding motion in the chip. This will result in a relative movement forward by the chip with respect to the fiber ends. The movement will stop once the set vertical force is reached from the increased resistance arising from the fiber butting on the waveguide couplers.

The uniformity of the fiber butting across the array is limited by the length uniformity of the fibers in the stub, the straightness of the fiber ribbon end cut. In turn, the fiber length uniformity is a function of the fiber cutting employed. A typical mechanical fiber ribbon cleave will not show a perfect line across the ribbon but a slight curve with some additional randomness from stub to stub. More accurate fiber cleaving can be achieved with CO2 lasers at the downside of increased cost. In practice, only a small number of fibers, the longest of the array, will actually touch their respective waveguide couplers. The remaining fibers will stop at a distance from the waveguide couplers as they were prevented by the already butted fibers from moving closer. Based on our experience, this will result in an optical-adhesive-filled gap between the fiber end and the waveguide coupler that can reach 10-20 um on some fibers.

Figure 4. Solution to fiber butting without in-plane movement of the pick-tip. The chip is vacuum-held on a sliding

base. A portion of the vertical force applied by the pick-tip is trigonometrically transformed into a fiber butting force, which effectively pushes the chip forward. (a) and (b) show the system after in-plane positioning. (c) and (d) show the

system during fiber lateral realignment in v-grooves. (e) and (f) show the system at fiber butting.

The optical performance is very sensitive to any lateral misalignment of fibers to waveguides but is somewhat tolerant of this gap due to the low divergence of the optical beam in an optical adhesive of the correct index of refraction. Nonetheless, the optical loss increases non-linearly with gap. Hence, fiber array butting is required to stay within the gap range needed for proper optical performance. A pick-and-place tool with ± 10 um accuracy requires positioning the longest fibers of an array at least 10 um from the waveguide coupler. Adding other tolerances we find that a substantial gap of at least 50 um would need to be optically tolerated on some fibers in assemblies without a butting mechanism. This would result in too large a penalty for most applications.

Experimental results The process of Fig. 4 was first implemented in a fully-

automated research and development die bonder (Finetech Fineplacer Femto). This tool was chosen for its flexibility and ease of programming but it also happens to show a much tighter placement accuracy than high-throughput tools. However, our process did not employ this characteristic so the results shown here are directly applicable to lower placement accuracy tools.

A picture of an assembled fiber stub to a photonic die is shown in Fig. 5. The length of the fibers in the stub can be customized to a given application. Most of the assemblies discussed here used a bare fiber length targeted at 5 mm. The 12 fiber ribbon is standard with a targeted fiber pitch of 250 um. The same 250 um pitch is used for the V-groove array on the photonic die. It is highly preferred to avoid a fiber pitch transformation, which would require much longer bare fibers and would significantly complicate the fabrication of the fiber stub with integrated lid. The V-groove length used in this example was of about 1.85 mm and could be significantly reduced in practice.

Cross-sections of a fiber stub assembled to a photonic chip are presented in Fig. 6. The cross-sections were taken across the V-groove array after the assembly process of Fig. 4. Some chipping is visible and is induced at sample cross-sectioning and polishing due to the alternating soft and hard materials in the stack. All 12 fibers in the array are well seated in the V-groove. The V-grooves are made purposefully deeper than required for the fibers to touch the V-groove sides only and provide sufficient bottom clearance to tolerate some level of particle contamination at assembly. The fiber core guiding the light in the fiber is visible in some cross-sections but not in others. This is to be expected due to the very low refractive index difference between the fiber core and the surrounding fiber cladding. The index contrast between the two is of only 0.3% so the visible contrast will be a strong function of illumination angle.

Figure 5. Top-down optical micrograph of a fiber stub

assembled to a photonic die. The length of the fibers can be customized to a given application.

Figure 6. Cross-sectional optical micrographs taken across the V-groove array after the assembly process of Fig. 4.

All fibers of the array are well seated in their respective V-grooves. Some chipping is seen and originates from sample preparation due to alternating soft and hard materials in the stack. The light-guiding fiber core is visible in

some fibers but not in others. This is expected from the strong sensitivity of the visible contrast to illumination angle, which results from the very small material contrast between the core and the surrounding cladding.

The fiber butting is shown in Fig. 7 with side-view and top-view cross-sections. The top-view cross-section was realized by polishing an assembly all the way to the middle height of the fibers. The fibers are butted on a suspended oxide membrane where the waveguide coupler is embedded. A key reason for the suspended design is to provide additional optical insulation to the silicon handle. The thickest buried oxide one is able to procure in a silicon-on-insulator wafer is of the order of 2-3 um. The optical mode in a single-mode optical fiber is around 10 um. Hence, a significant amount of power would be caught by the silicon handle if additional optical insulation was not provided near the fiber edge. The waveguide coupler is designed to shrink the mode to the typical sub-micron mode size used on wafer so the additional optical insulation is only required in the fiber-edge area. The cavity in between the suspended membrane and the silicon wafer handle is filled with optical adhesive at assembly. Venting holes etched through the membrane facilitate the adhesive fill, as seen in Fig. 7(b). The optical performance of a waveguide coupler embedded in a suspended waveguide was found excellent with -1.3dB peak efficiency and only 0.8dB penalty over a 100 nm wavelength bandwidth and all polarizations [7]. As the side view cross-section in Fig. 7(a) is taken through the middle of the fiber, an adhesive gap under the fiber is expected as correspondingly shown in Fig. 6. As mentioned above, the fibers contact the sidewalls of the V-grooves only and extra clearance is designed in between the fiber and the V-groove bottom for particle tolerance at assembly.

The front and corners of the suspended membrane are designed curved to improve tolerances to the cleave angle of individual fibers and to maximize mechanical strength. The

mechanical robustness of the suspended design is addressed below. As discussed above, the fiber to waveguide gap is limited by the fiber length uniformity across the fiber array. Hence, some fibers will be in contact with the suspended membrane and some will not. Fig. 7 shows fibers with sufficiently small gap to the waveguide coupler to be considered in contact. In addition to the length uniformity across the fiber array, each individual fiber shows variation in cleave angle of ~1 degree from the desired normal cleave to light propagation. This can further increase the gap in the light guiding portion by pushing the contact point towards the fiber edge but can be mitigated with a curved membrane front as shown here.

Considerations An analysis of fiber lateral re-alignment is shown in Fig. 8.

The re-alignment range of a V-groove is schematically presented in Fig. 8(a) for the current design. Assuming that the tangent to the fiber contacting the V-groove edge should form an angle of at least 30 degrees from the horizontal for proper re-alignment into the V-groove, we find a re-alignment capability exceeding +/- 40 um in our current V-groove design. Some of this range needs to be set aside for placement inaccuracy. Assuming a pick-and-place placement accuracy of 10 um, we find that the fibers in the fiber array must be maintained by the polymer lid to within +/- 30 um of their

Figure 7. Two polished cross-sections showing fiber

butting on the waveguide coupler, which is embedded in a suspended oxide membrane. A side view is shown in (a) and a top view, polished into the fiber and the suspended membrane, is shown in (b). The same region but prior to

fiber assembly is shown in Fig. 9(a).

Figure 8. Schematics of fiber lateral re-alignment in V-

groove. The maximum re-alignment range is shown in (a). The desired structure after re-alignment is shown in (b) with the coordinate system used in Fig. 9 and Table 1.

expected position for successful transversal self-alignment in the V-grooves.

Once seated in the V-grooves, the fiber to waveguide alignment accuracy is limited by the fiber and photonic chip fabrication accuracy. This is illustrated in Fig. 8(b). A Monte Carlo tolerance analysis is presented in Fig 9. Additional insight is presented in Table 1, where the assumptions and the geometrical impacts of error parameters on x and y alignment accuracy are given. The fiber non-circularity and fiber core non-concentricity create alignment errors whose decomposition in x and y components is randomized and cannot be a priori stated so a range is provided instead. The V-grooves are fabricated by first etching the V-groove pattern in the thick dielectric stack on top of the silicon handle wafer. This dielectric stack then acts as a hardmask for wet anisotropic etching of silicon. TMAH was used here as the anisotropic

etchant due to its compatibility with CMOS. Typical fabrication error assumptions were used for V-groove accuracy based on i-line stepper lithography with thick resist and thick dielectric etch. The optical fiber tolerances were based on the publically available specification sheet for the SMF28e+ fiber [8].

The various error contributions in Table 1 are treated as statistically uncorrelated, which is acceptable for the purpose here of estimating the range of fiber core to waveguide misalignment. Strictly speaking, some low level of correlation could be present. For instance, an issue with TMAH concentration at anisotropic etching would affect both the etch depth and selectivity. However, in production environments, the etch rate is closely monitored so the etch depth and selectivity could be mostly de-correlated by human action if the etch time is adjusted to compensate for a change in etch rate. A

Figure 9. Monte Carlo tolerance analysis of fiber-core to waveguide-coupler alignment. A sample size of 10,000

random error combinations is plotted along with the marginal x and y distributions. The total misalignment is shown in (d) with broken down contributions shown in (a) to (c).The impact of fiber-core non-concentricity is

shown in (a). The impact of fiber cladding dimensional variability in diameter and non-circularity is shown in (b). The impact of variability in V-groove fabrication is shown in (c). The total maximum misalignment expected is of

~1.3 um, well suited for photonics. A breakdown of assumptions and geometrical impacts is shown in Table 1.

sample size of 10,000 random error combinations was used in the Monte Carlo analysis. The misalignments resulting from (a) fiber core non-concentricity, (b) fiber cladding dimensional variability and non-circularity, and (c) V-groove fabrication uncertainty were separately computed and then summed. The result is shown in Fig. 9(d). The alignment at volume manufacturing is expected to stay within ~1.3 um, which is well suited for single-mode optical coupling.

In addition to fiber alignment, another yield consideration is the mechanical reliability of butting fibers on a suspended oxide membrane. The details of the design of the suspended membrane with embedded waveguide coupler is shown in Fig.10(a), a top-down optical micrograph taken prior to assembly with focus set at the waveguide coupler layer. The maximum butting force that such membrane can survive was experimentally measured using the setup of Fig. 10(b). A shear test tool was used to push a piece of fiber placed in a V-groove onto the suspended membrane. The resulting data is shown in Table 2. We see that a single suspended membrane can survive tens of gram.force, which is ample for our process. In addition, the strength increases substantially with membrane thickness, as expected, so the membrane could be further strengthened at the cost of moderately more expensive wafer processing.

There is a tradeoff in the sliding base angle used for the butting motion. A shallow sliding angle (with respect to the horizontal plane) will result in strong fiber re-alignment force to push the fibers in the grooves but will also decrease the butting force component and increase the friction of the fiber movement in the V-grooves. A larger sliding angle, will increase the butting force and will reduce the friction force but will also reduce the re-alignment force of the fibers entering the

v-grooves. The angle must be small enough for repeatable fiber lateral re-alignment and large enough for repeatable butting without generating excessive force that could damage the receiving waveguide couplers.

The fiber-to-lid adhesion strength needs to be considered in tandem with the sliding angle. The adhesion strength must be

Figure 10. Measuring the strength of a suspended

membrane. The details of the suspended membrane with embedded waveguide is shown in a top-down optical

micrograph in (a). A picture of the measurement setup is shown in (b). A shear stress tool was employed to push an optical fiber against the suspended membrane and record

the breaking force.

Table 1. Additional insights into tolerance analysis on fiber-core to waveguide alignment

Tolerance

assumption Geometrical factor* Worst impact

x y x y Fiber tolerances Diameter ± 0.70 um 0 0.87 0 um 0.61 um

Non-circularity ± 0.87 um 0 to 0.29 0.15 to 0 0 to 0.25 um 0.13 to 0 um

Core non-concentricity ± 0.50 um 0 to 1 1 to 0 0 to 0.5 um 0.5 to 0 um V-groove tolerances

Alignment to waveguide ± 0.25 um 1 0 0.25 um 0 um

Mask open width ± 0.75 um 0 0.71 0 um 0.53 um

Mask open overetch ± 0.05 um 0 1 0 um 0.05 um

Anisotropic etch selectivity ± 5 (off 37:1) 0 0.11 0 um 0.55 um

Etch depth (100) ± 3.75 um 0 0.05 0 um 0.19 um

* Factors calculated at tolerance limit for illustrative purpose only. The impact of some errors is non-linear and the combination of some errors is non-linear as well so the Monte Carlo analysis uses direct geometrical calculations without pre-defined geometrical factors.

Table 2. Measured membrane longitudinal breaking force

Main material Membrane thickness

Min Max

PECVD* SiO2 TEOS precursor

8 um 20 g.f 44 g.f

PECVD SiO2 silane precursor

11 um 75 g.f 90 g.f

* PECVD stands for plasma enhanced chemical vapor deposition

strong enough to overcome the force resulting from the friction between fibers and grooves at the butting step. Otherwise, the fibers would be pushed by the forward motion of the chip and slide at the fiber-to-lid interface. On the other hand, a fiber that is too rigidly fixed to a lid will require a large lateral re-alignment force. In short, a large adhesion strength between fibers and lid should be paired with a small sliding angle and a large vertical assembly force. Correspondingly, a small adhesion strength between fibers and the lid should be paired with a large sliding angle and a small vertical assembly force. The upper limit of the vertical assembly force at a given sliding angle corresponds to the maximum butting force allowed by the receiving suspended membrane. The larger the sliding angle, the smaller the upper limit of the vertical assembly force is for a given butting force.

Conclusions We have described a novel approach for parallelized optical

fiber to photonic chip assembly. The approach is compatible with high-throughput pick-and-place tooling and the current implementation can assemble one-dimensional fiber arrays of any size to a V-groove array on a photonic chip.

We have identified the main issues with enabling standard high-throughput pick-and-place tools for photonic assembly. First, a lid is assembled to the bare fiber array to allow vacuum pick-tip handling. Second, a V-groove array integrated on the photonic chip is used for lateral self-alignment of fibers. Third, a sliding chip mount is used for fiber butting on waveguide couplers by trigonometrically transforming part of the vertical assembly force into a horizontal butting force.

We have demonstrated the concept experimentally by assembling 12-fiber arrays. We have shown that all fibers were well re-aligned and seated in the V-grooves with fiber butting limited by the fiber length cleave uniformity. We have presented an in-depth Monte Carlo tolerance analysis demonstrating an expected misalignment of less than 1.3 um between fiber-core and waveguide-coupler at manufacturing despite up to ~40 um initial misalignment. The final alignment range is well within the required 1-2 um alignment for single-mode photonics. As a final point, we have investigated yield of fiber butting on the suspended membrane waveguide coupler. Experimental measurements have shown a breaking strength of tens of gram.force, which is ample for our process.

The approach described here could ease large scale silicon photonics deployment by significantly improving on optical I/Os cost, scalability in fibers per chip, and scalability in manufacturing volume.

Acknowledgments We would like to thank Guy Brouillette for operating the

die bonding tool and Genevieve Beaulieu for acquiring the cross-sectional micrographs. We would also like to thank the staff of the Microelectronic Research Laboratory (MRL) at the IBM T.J. Watson Research Center where the photonic chips were fabricated.

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