Pixel Sensor Isolation Structure for X-Road Etch Depth Control

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Solution Overview

Problem

The etch rate for deep trench isolation (DTI) structures in CMOS image sensors is uneven, particularly at X-roads, leading to increased trench depth loading and potential damage to floating diffusion regions during etching cycles, resulting in performance issues and electrical shorts.

Innovation Solution

A self-aligned plug layer is formed using an organic compound to protect the X-road area during etching cycles, allowing for controlled etching of other areas without damaging the floating diffusion region, thereby reducing etch depth and preventing electrical failures.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If deep trench isolation structures are etched to electrically isolate photodiodes, then electrical isolation between pixel sensors is improved, but uneven etch rate at X-roads causes increased trench depth loading and potential damage to floating diffusion regions

Engineering Contradiction:

Improveelectrical isolationVSAvoidetch depth control

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The etching process is divided into multiple cycles with a protective plug layer that segments the exposure of different trench regions. The plug layer is selectively removed at X-roads to allow controlled etching in subsequent cycles, preventing over-etching while maintaining isolation effectiveness.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A protective plug layer is formed in the trench before the complete etching process. This preliminary protective structure prevents direct exposure of the floating diffusion region to the etchant, allowing the trench to be etched to the required depth without damaging sensitive underlying structures.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If multiple etching cycles are performed to achieve sufficient trench depth, then electrical isolation is improved, but the floating diffusion region becomes more susceptible to damage from prolonged etching exposure

Engineering Contradiction:

Improveelectrical isolationVSAvoidetching damage

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The protective plug layer acts as a cushioning barrier that absorbs and protects the floating diffusion region from the harmful effects of multiple etching cycles. This pre-positioned protective structure allows the etching process to proceed through multiple cycles without directly exposing the sensitive region to etchant damage.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

Solution Approach 2:

The protective plug layer serves as an intermediary structure between the etchant and the floating diffusion region. It mediates the interaction by providing a sacrificial barrier that can be selectively removed, allowing controlled etching while protecting the underlying sensitive structures from direct contact with the etchant.

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS20240120363A1Pixel sensor isolation structures and methods of forming the same

Publication Date: 2024.04.11 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

A self-aligned plug may be formed between deep trench isolation (DTI) etching cycles. Accordingly, etch depth in areas of a pixel sensor with large CDs (e.g., at an X-road) is reduced, which prevents trench loading. As a result, a floating diffusion (FD) region, associated with photodiodes of the pixel sensor, is not damaged during the DTI etching cycles. Reduced chances of damage to the FD region improves performance of the pixel sensor and prevents electrical shorts and failures, which increases yield and conserves time and raw materials used in forming the pixel sensor.