Secondary Electron Injection NROM Cell Programming

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Solution Overview

Problem

Nitride read-only memory (NROM) cells face challenges with secondary electron injection, which can lead to 'second bit' and retention after cycling problems due to wide electron distribution, and require high programming currents and numerous bitline contacts, resulting in scaling difficulties and resistance issues.

Innovation Solution

Employing secondary electron injection (SEI) with low wordline voltage, negative substrate voltage, and specific shallow and deep implants to enhance secondary electron generation and injection close to the junction, reducing channel hot electron injection and optimizing pocket implants to control electron distribution and retention.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Use of energy by moving object

If secondary electron injection is used for programming NROM cells, then programming current requirements are reduced, but electron distribution becomes wide causing second bit and retention problems

Engineering Contradiction:

Improveprogramming currentVSAvoidelectron distribution width

Core Design Contradiction:

Use of energy by moving objectVSManufacturing precision

Solution Approach 1:

The patent introduces a graded pocket implant structure where the implant concentration varies spatially - higher concentration near the drain to focus electron injection and lower concentration away from the drain to reduce spread. This local variation in implant quality narrows the electron distribution width while maintaining the low programming current benefit of secondary electron injection

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The patent modifies the physical parameters of the pocket implant region by adjusting the implant concentration gradient and depth profile. By changing these parameters, the electron distribution is controlled to be narrower, preventing second bit effects while retaining the energy efficiency of secondary electron injection programming

Inventive Principle:

Principle #35Parameter changes

2Manufacturing precision

If channel hot electron injection is used for programming, then electron distribution is narrow, but high programming currents and numerous bitline contacts are required

Engineering Contradiction:

Improveelectron distribution widthVSAvoidprogramming current

Core Design Contradiction:

Manufacturing precisionVSPower

Solution Approach 1:

The patent introduces a graded pocket implant as an intermediary structure that mediates between the two injection mechanisms. It enables secondary electron injection (which is more energy-efficient) to achieve the narrow electron distribution normally associated with channel hot electron injection, thus reducing programming current while maintaining precision

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

By changing the implant concentration parameters in the pocket region, the patent creates conditions that favor secondary electron generation and injection while confining the electron distribution. This parameter optimization reduces the power requirement for programming compared to conventional channel hot electron injection

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If secondary electron injection is used, then fewer bitline contacts are needed, but retention after cycling deteriorates

Engineering Contradiction:

Improvenumber of bitline contactsVSAvoidretention after cycling

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The graded pocket implant creates local quality variations where the implant concentration is optimized in specific regions to ensure electrons are injected and retained in the correct charge storage area. This prevents the wide distribution that causes retention problems while maintaining the simplified bitline contact structure enabled by secondary electron injection

Inventive Principle:

Principle #3Local quality

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 reduces programming current and voltage requirements, narrows electron distribution, and improves retention and punchthrough, allowing for more efficient and reliable NROM cell operation with fewer bitline contacts and relaxed power supply constraints.

Implementation Method 1

utilizing secondary electron injection to perform a given electron injection operation

Methodology Applied

Scientific EffectSecondary electron injection:

Implementation Method 2

enhancing secondary electron generation and injection close to the junction, reducing channel hot electron injection

Methodology Applied

Scientific EffectChannel hot electron injection:

Data Source

PatentUS7808818B2Secondary injection for NROM

Publication Date: 2010.10.05 SAIFUN SEMICON LTD

AI summary

Secondary electron injection (SEI) is used for programming NVM cells having separate charge storage areas in an ONO layer, such as NROM cells. Various combinations of low wordline voltage (Vwl), negative substrate voltabe (Vb), and shallow and deep implants facilitate the process. Second bit problems may be controlled, and retention and punchthrough may be improved. Lower SEI programming current may result in relaxed constraints on bitine resistance, number of contacts required, and power supply requirements.