Alignment-tolerant interfacing of a photonic integrated circuit using back side etched silicon microlenses

Abstract

Directly interfacing a photonic integrated circuit allows at best an alignment tolerance of a few micrometer due to the small dimensions of optical (coupling) features on chip, but when using microlenses integrated on the substrate-side, alignment tolerances for interfacing the chips can greatly be relaxed. This is demonstrated on a 750 μm thick chip with standard grating couplers (operation wavelength around 1550 nm). Low roughness silicon microlenses were realized by transferring reflowed photoresist into the silicon substrate using reactive ion etching. The microlens allows interfacing the chip from the backside with an expanded beam, drastically increasing lateral alignment tolerances. A 1 dB alignment tolerance of ±8 μm and ±11 μm (along and perpendicular to the grating coupler direction, respectively) was experimentally found when a 40 μm mode field diameter beam was used at the input.

Expanded-Beam Backside Coupling Interface for Alignment-Tolerant Packaging of Silicon Photonics

“…Although the focus of the paper is to investigate the alignment tolerances, it is interesting to tabulate the measured and estimated optical loss distribution across the expanded beam interface, listing the corrective measures needed for further improvement (Table I). In order to investigate the tolerances arising out of the placement of the microlens with respect to the output grating coupler, if it were to be monolithically fabricated on the chip backside [17], a lateral alignment of the photonic chip was performed with respect to the microlens substrate, keeping the output fiber position fixed. A ±2.5 µm lateral 1-dB alignment tolerance was measured between the grating and the microlens (Fig.…”

“…After UVpatterning and development of the exposed resist, the obtained cylindrical structures were thermally reflowed, followed by a hard-bake. The reflown microlens profiles in resist were then transferred to silicon by a reactive-ion etch (RIE) process with an optimized composition of SF6 and O2 [17]. A 3D-surface white light profilometry was performed to verify the fabricated microlenses, which had a diameter of 240.7 µm, lens height (sag) of 18.3 µm and a radius of curvature of 480 µm, which corresponded very closely to the desired specs obtained from the beam propagation simulations in the ray-trace model (Fig.…”

“…Therefore, we have proposed to integrate microlenses at the backside of an integrated photonics chip to collimate the expanded beam resulting in a relaxed tolerance to both lateral and longitudinal misalignment ( Fig. 2a) [16], [17]. Additionally, by using such a through-substrate coupling scheme, the device side (top-side) of the chip remains accessible for any additional fiber-array packaging and die stacking for 2.5D/3D electrooptic integration (Fig.…”

“…Finally, this approach would be beneficial for various sensing applications since the device topside can be kept clear to allow the photonic circuit to interact with biomarkers or trace gas molecules. Although we have presented initial results for monolithically integrated microlenses in combination with C-band grating couplers earlier [17], the current paper (i) targets O-band grating couplers and (ii) aims at investigating in detail the tolerances and individual contributions to the coupling loss of such a backside expanded beam interface. This is achieved by hybrid integrating a silicon microlens and a photonics chip consisting of a downward directionality based O-band grating coupler.…”

Through-substrate coupling elements for silicon-photonics-based short-reach optical interconnects

“…5). Although a monolithic solution for integrating the microlenses is the ultimate goal, 10 we discuss here an intermediate step of hybrid-integrating a photonics chip with polymer microlens fabricated on a separate dual-side polished Si substrate. 11 Although the concept of an expanded beam collimation itself can be realized from the topside of the chip as has been described comprehensively already, 12 yet a through-substrate coupling allows for an alternative approach of doing face-up integration of the photonics chip and helps provide easy access to the device-side for advanced packaging technologies.…”