Backside-Deposited Grating Coupler Mirror for Low-Loss Optical Interconnects

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Solution Overview

Problem

Existing grating couplers in optical communications suffer from high loss and limited bandwidth due to the complexity and cost of fabricating bottom mirrors, often requiring specialized wafers and recalibration of foundry processes, which results in low reflectivity and poor uniformity.

Innovation Solution

A method involving etching a trench around the grating coupler, undercutting the silicon, and depositing a mirror from the backside onto the oxide layer, allowing for improved coupling efficiency and bandwidth without compromising the mechanical integrity of photonic integrated circuits.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a bottom mirror is fabricated using specialized wafers and recalibrated foundry processes, then the reflectivity and coupling efficiency are improved, but the manufacturing complexity and cost increase significantly

Engineering Contradiction:

Improvecoupling efficiencyVSAvoidfabrication complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

Instead of depositing the bottom mirror from the front side using specialized wafers and recalibrated processes, the patent inverts the approach by accessing the backside of the wafer to deposit the mirror from below. This eliminates the need for specialized front-side processing while achieving the same reflective function, thereby reducing fabrication complexity and cost while maintaining coupling efficiency

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The fabrication process is segmented into standard front-side processing steps followed by backside access and mirror deposition. By separating the mirror deposition from the front-side processing sequence and using standard foundry processes for both sides, the patent reduces the overall manufacturing complexity while maintaining the reflective function needed for high coupling efficiency

Inventive Principle:

Principle #1Segmentation

2Reliability

If a bottom mirror is fabricated using specialized wafers and recalibrated foundry processes, then the reflectivity is improved, but the manufacturing cost increases

Engineering Contradiction:

ImprovereflectivityVSAvoidmanufacturing cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent inverts the conventional approach by depositing the bottom mirror from the backside using standard foundry processes rather than requiring specialized front-side processing. This eliminates the need for expensive specialized wafers and recalibrated equipment while achieving the required reflectivity, thereby significantly reducing manufacturing cost

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The patent employs standard, readily available foundry processes and materials for mirror deposition rather than requiring expensive specialized components. By using conventional process tools and materials that can be produced at scale, the patent reduces the per-unit manufacturing cost while maintaining the functional performance of the bottom mirror

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

3Ease of manufacture

If conventional grating coupler fabrication is used, then the manufacturing process is simpler, but the bandwidth is limited

Engineering Contradiction:

Improvefabrication simplicityVSAvoidbandwidth

Core Design Contradiction:

Ease of manufactureVSAdaptability or versatility

Solution Approach 1:

By accessing the backside of the wafer to deposit the bottom mirror, the patent enables enhanced optical coupling and wider bandwidth without complicating the front-side fabrication process. The backside access allows for optimized mirror positioning and reflectivity control that expands the operational bandwidth while maintaining fabrication simplicity on the front side

Inventive Principle:

Principle #13The other way round (Inversion)

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 enhances coupling efficiency by 2-4 dB and widens the bandwidth by approximately 30% while reducing fabrication costs and maintaining single-crystalline waveguide integrity.

Implementation Method 1

depositing a mirror on the bottom of the grating coupler

Methodology Applied

Scientific EffectReflection: Reflection

Data Source

PatentUS8883018B2Low-loss, wide-band grating coupler and method of making same

Publication Date: 2014.11.11 ACACIA TECH INC

AI summary

A method for fabricating a grating coupler having a bottom mirror in a semiconductor wafer including etching a trench from a top surface of a wafer and around a grating coupler formed in the wafer; etching a void underneath the grating coupler; etching a via into the void from the backside of the wafer; and depositing a mirror on the bottom of the grating coupler. Alternatively, additional oxide may be deposited on the bottom of the grating coupler prior to the deposition of the mirror such that a desirable oxide thickness on the bottom is achieved.