Shallow Trench Air Gaps for NAND Flash Isolation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
As dimensions of shallow trench isolation (STI) structures in NAND flash memories decrease, providing sufficient isolation between neighboring NAND strings becomes challenging, leading to potential damage to interpoly dielectric and gate dielectric layers during isotropic etching, which can affect memory cell characteristics.
Innovation Solution
The formation of air gaps in STI trenches using isotropic etching, with protective sleeves to prevent exposure of word lines and interpoly dielectric to etching, and using polysilazane as isolating material to protect gate dielectric, ensuring effective isolation between NAND strings while minimizing damage to these layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If isotropic etching is performed to form air gaps in STI trenches, then isolation between NAND strings is improved, but damage to interpoly dielectric and gate dielectric layers increases
Solution Approach 1:
A protective sleeve made of silicon nitride is introduced as an intermediary layer between the isotropic etchant and the sensitive dielectric structures. The sleeve extends below the substrate surface and acts as a barrier that allows the etchant to access the STI trench for air gap formation while preventing direct contact with and damage to the interpoly dielectric and gate dielectric layers.
Solution Approach 2:
The protective sleeve is formed prior to the isotropic etching process, extending below the substrate surface in advance. This preliminary positioning ensures that when the isotropic etchant is applied, the sensitive dielectric layers are already protected, preventing damage before it can occur.
2Area of stationary object
If STI trench width is reduced to increase memory density, then area utilization is improved, but isolation effectiveness between neighboring NAND strings deteriorates
Solution Approach 1:
Air gaps (gaseous medium) are formed within the STI trenches to provide electrical isolation between neighboring NAND strings. These air gaps replace traditional solid or liquid fill materials, providing effective insulation while occupying minimal space, thus maintaining isolation effectiveness even when trench dimensions are reduced for higher density.
3Object-affected harmful factors
If protective sleeves are formed to protect dielectric layers, then damage to interpoly dielectric is reduced, but process complexity increases
Solution Approach 1:
The protective sleeve serves multiple functions simultaneously: it protects the interpoly dielectric from etchant damage, defines the lower boundary for isotropic etching to enable air gap formation, and provides structural support during subsequent processing steps. This multi-functionality reduces the need for additional separate protective layers or process steps.
Solution Approach 2:
The protective sleeve is integrated within the existing STI trench structure, nesting the protection function within the isolation structure itself. The sleeve is positioned concentrically within the trench, allowing the STI structure to provide both isolation and protection functions through a unified nested design rather than requiring separate external protective elements.
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 provides reliable isolation between NAND strings, reducing damage to interpoly dielectric and gate dielectric layers, thereby enhancing the integrity and performance of memory cells by maintaining the physical dimensions and reducing charge trapping sites.
Implementation Method 1
Air gaps may be formed in STI trenches using isotropic etching
Implementation Method 2
using polysilazane as isolating material to protect gate dielectric
Data Source
AI summary
A method of forming a NAND flash memory includes etching between word lines to expose isolation material in shallow trench isolation (STI) trenches while active areas between word lines remain covered, then forming protective sleeves at locations over exposed isolation material. Subsequently, with the protective sleeves in place, isotropic etching of isolation material forms an air gap extending continuously between the protective sleeves along an individual STI trench.