Split Level STI Body Contact for SOI MOSFET Area Efficiency

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Solution Overview

Problem

Conventional semiconductor-on-insulator (SOI) devices face issues with high parasitic capacitance and charge retention due to floating body contacts, which affect current-sensitive circuit applications and increase power dissipation.

Innovation Solution

The introduction of a body contact structure using two levels of shallow trench isolation (STI) with different thicknesses, eliminating the need for T-shaped or H-shaped polysilicon structures, allows for direct contact with the SOI extension and reduces area requirements, thereby minimizing parasitic capacitance and charge retention.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If T-shaped or H-shaped polysilicon structure is used for body contact, then body contact is achieved and threshold voltage can be changed, but layout area increases and power dissipation increases

Engineering Contradiction:

Improvethreshold voltage controlVSAvoidlayout area

Core Design Contradiction:

ReliabilityVSArea of stationary object

Solution Approach 1:

The body contact structure is segmented into multiple regions: a first body contact region with first conductivity type and a second body contact region with second conductivity type. This segmentation allows independent control of threshold voltages for n-channel and p-channel transistors while reducing the overall layout area compared to conventional T-shaped or H-shaped polysilicon structures.

Inventive Principle:

Principle #1Segmentation

2Reliability

If T-shaped or H-shaped polysilicon structure is used for body contact, then body contact is achieved, but parasitic capacitance increases

Engineering Contradiction:

Improvebody contact functionalityVSAvoidparasitic capacitance

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The invention extracts and eliminates the polysilicon gate structure from the body contact region. By using implanted dopant regions directly as body contacts instead of polysilicon structures, the parasitic capacitance associated with polysilicon-gate-body overlap is removed, while maintaining effective body contact functionality.

Inventive Principle:

Principle #2Taking out (Extraction)

3Device complexity

If body is kept floating to simplify structure, then structure is simpler, but charge retention occurs and interferes with subsequent device use

Engineering Contradiction:

Improvestructure complexityVSAvoidcharge retention

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The body contact structure serves dual functions: it provides electrical contact to control threshold voltage and simultaneously drains accumulated charge from the body to the substrate. The implanted dopant regions with conductivity type opposite to the channel create inherent charge drainage paths, eliminating charge retention issues without requiring external body contact structures.

Inventive Principle:

Principle #25Self-service

Data Source

PatentUS8680617B2Split level shallow trench isolation for area efficient body contacts in SOI MOSFETS

Publication Date: 2014.03.25 TAIWAN SEMICONDUCTOR MANUFACTURING CO LTD

AI summary

Disclosed is an SOI device on a bulk silicon layer which has an FET region, a body contact region and an STI region. The FET region is made of an SOI layer and an overlying gate. The STI region includes a first STI layer separating the SOI device from an adjacent SOI device. The body contact region includes an extension of the SOI layer, a second STI layer on the extension and a body contact in contact with the extension. The first and second STI layers are contiguous and of different thicknesses so as to form a split level STI.