Flip Chip Optical Interconnects Using GRIN Lenses
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Solution Overview
Problem
Conventional optical interconnection methods at the chip level face limitations such as low bandwidth density, high coupling losses, and stringent alignment requirements, which increase fabrication costs and reduce ease of adoption.
Innovation Solution
The use of a graded index (GRIN) lens and cross-taper couplers on a chip carrier to optically couple photonic integrated circuits (PICs) with external circuitry, allowing for high bandwidth density, low coupling losses, and increased alignment tolerance, leveraging existing semiconductor fabrication processes and tools.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional optical fibers with thick cladding are used for interconnection, then crosstalk between adjacent fibers is prevented, but bandwidth density is limited due to the bulkiness of the fibers
Solution Approach 1:
The patent extracts the essential light-guiding function from conventional optical fibers by removing the thick cladding structure. Instead, it uses evanescent field coupling between adjacent waveguides on the chip carrier, where the optical signal is confined to sub-wavelength dimensions, enabling high-density integration without crosstalk
Solution Approach 2:
The patent transitions from three-dimensional fiber routing to two-dimensional planar waveguide integration. By confining optical modes in the vertical dimension through sub-wavelength thickness, the system achieves high bandwidth density in the horizontal plane, effectively using dimensional transformation to overcome spatial constraints
2Loss of energy
If high precision alignment is used to reduce coupling losses between optical fibers and waveguides, then coupling losses decrease, but fabrication costs increase and ease of adoption reduces
Solution Approach 1:
The patent performs preliminary action by pre-configuring the waveguide geometries, materials, and spacing during the chip carrier fabrication process itself. The evanescent coupling structures are built-in with precise dimensions established through standard semiconductor manufacturing, eliminating the need for post-fabrication alignment adjustments
Solution Approach 2:
The optical coupling is achieved through self-aligned evanescent field interaction between adjacent waveguides. The coupling efficiency is determined by the inherent physical dimensions and material properties of the waveguides rather than external alignment, making the system self-adjusting and robust to manufacturing variations
3Reliability
If conventional fiber pigtailed couplers are used for optical interconnection, then optical signal transmission is achieved, but device complexity increases and alignment tolerance decreases
Solution Approach 1:
The patent merges the optical waveguide, coupling structure, and mounting substrate into a single integrated chip carrier. The evanescent coupling waveguides are formed directly on the chip carrier substrate, eliminating separate fiber pigtailed couplers and reducing the overall interconnection structure to a compact, monolithic device
Solution Approach 2:
The chip carrier serves multiple functions simultaneously: it provides mechanical support, electrical connections via ball grid array, optical waveguiding through evanescent coupling structures, and precise alignment registration features. This multi-functionality eliminates the need for separate specialized components for each function
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables high-throughput optical interconnects with improved bandwidth density, reduced coupling losses, and increased alignment tolerance, facilitating the integration of PICs into existing frameworks while being cost-effective and compatible with CMOS processes.
Implementation Method 1
The GRIN lens has a curved end, a tapered end, and a refractive index that varies with height
Implementation Method 2
The first tapered end of the first waveguide is in optical communication with the tapered end of the GRIN lens
Data Source
AI summary
Optical interconnects can offer higher bandwidth, lower power, lower cost, and higher latency than electrical interconnects alone. The optical interconnect system enables both optical and electrical interconnection, leverages existing fabrication processes to facilitate package-level integration, and delivers high alignment tolerance and low coupling losses. The optical interconnect system provides connections between a photonics integrated chip (PIC) and a chip carrier and between the chip carrier and external circuitry. The system provides a single flip chip interconnection between external circuitry and a chip carrier using a ball grid array (BGA) infrastructure. The system uses graded index (GRIN) lenses and cross-taper waveguide couplers to optically couple components, delivers coupling losses of less than 0.5 dB with an alignment tolerance of ±1 μm, and accommodates a 2.5× higher bandwidth density.