Shallow Trench Isolation Air Gaps for NAND Memory Leakage Reduction

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Solution Overview

Problem

Current fabrication techniques for high-density semiconductor devices, such as NAND flash memory, face challenges in reducing parasitic capacitance and word line-active area leakage as feature sizes decrease, affecting storage capacity and reliability.

Innovation Solution

The formation of shallow trench isolation (STI) air gaps within the semiconductor substrate, including methods to create air gaps in the interpoly dielectric (IPD) layer above STI air gaps, helps reduce parasitic capacitance and leakage by using dielectric materials like polysilazane and silicon dioxide, and selectively removing portions of the IPD layer to optimize electrical isolation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Quantity of substance

If feature sizes are reduced to increase storage density, then storage capacity increases, but parasitic capacitance and word line-active area leakage increase

Engineering Contradiction:

Improvestorage densityVSAvoidparasitic capacitance and leakage

Core Design Contradiction:

Quantity of substanceVSReliability

Solution Approach 1:

Air gaps are formed by removing dielectric material from specific regions between memory cell strings, extracting the harmful dielectric that causes parasitic capacitance while maintaining the physical structure needed for high-density storage

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The structure transitions from uniform dielectric filling to non-uniform composition with air gaps strategically positioned in inter-string isolation regions, creating local electrical isolation where needed while maintaining dielectric continuity in other areas

Inventive Principle:

Principle #3Local quality

2Ease of manufacture

If conventional STI structures are used without air gaps, then manufacturing is simpler, but cell-to-cell crosstalk and parasitic capacitance are higher

Engineering Contradiction:

ImproveSTI formation processVSAvoidcell-to-cell crosstalk and parasitic capacitance

Core Design Contradiction:

Ease of manufactureVSObject-affected harmful factors

Solution Approach 1:

The continuous dielectric material in the STI structure is segmented into discrete regions separated by air gaps, dividing the isolation function into multiple zones that provide both mechanical support and electrical isolation

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Air gaps serve as intermediary regions between adjacent memory cell strings, providing electrical isolation and reducing capacitive coupling while maintaining the physical continuity of the STI structure for mechanical support

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentUS9779983B2Methods for forming air gaps in shallow trench isolation trenches for NAND memory

Publication Date: 2017.10.03 SANDISK TECHNOLOGIES LLC

AI summary

A method of forming a shallow trench isolation trench in a semiconductor substrate is described. The method includes forming a trench in a region of the substrate, forming a first dielectric material in the trench, forming a second dielectric material above the first dielectric material, forming a first air gap in the first dielectric material in the trench, and forming a second air gap in the second dielectric material above the first air gap.