MONOS Memory Charge Storage Film Segmentation for Retention
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Solution Overview
Problem
The retention characteristic of memory cell transistors with a MONOS type structure in NAND flash memory devices deteriorates due to damage during the processing of tunnel insulating and charge storage films, leading to charge shifting between adjacent cells.
Innovation Solution
A nonvolatile semiconductor memory device structure that includes a tunnel insulating film, a charge storage film with a degraded charge film formed by diffusing an oxidizing agent from the element isolation insulating film, which separates adjacent charge films and prevents charge shifting, and a floating gate type memory cell with an inter-gate multilayer film having a high dielectric constant material on its side surface to reduce capacitance coupling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If RIE is used to form element isolation trenches and process tunnel insulating film and charge storage film simultaneously, then manufacturing efficiency is improved, but damage occurs to the tunnel insulating film and charge storage film causing deterioration of retention characteristic
Solution Approach 1:
The patent divides the charge storage film into two distinct parts: a first charge storage film formed before element isolation trenches, and a second charge storage film formed after. This segmentation allows the first film to be protected from RIE damage while the second film can be processed efficiently, resolving the contradiction between manufacturing efficiency and retention characteristic.
Solution Approach 2:
The first charge storage film is formed in advance before the element isolation trenches are created. This preliminary action ensures that the charge storage film is already in place and protected when the trenches are formed via RIE, preventing damage to the film during the isolation process while maintaining manufacturing efficiency.
2Reliability
If element isolation trenches are formed before depositing charge storage film, then damage to tunnel insulating film is avoided, but charge storage films of adjacent memory cell transistors become joined causing charge shifting
Solution Approach 1:
The patent applies different processing sequences to different regions: the first charge storage film is formed over the entire surface including future trench locations, while the second charge storage film is formed only in specific regions after trench formation. This local differentiation allows adjacent films to be separated by trenches while maintaining continuous charge storage functionality within each cell, preventing charge shifting.
Solution Approach 2:
By dividing the charge storage film formation into two stages with trench formation in between, the patent creates physical separation (segmentation) between adjacent memory cell charge storage films. This prevents the films from joining and eliminates the charge shifting problem while still protecting the tunnel insulating film from RIE damage.
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
Improves the retention characteristic by preventing charge shifting between adjacent memory cell transistors and reducing leakage current, enhancing the reliability and storage stability of NAND flash memory devices.
Implementation Method 1
a degraded charge film formed by diffusing an oxidizing agent from the element isolation insulating film
Implementation Method 2
oxidize the charge storage film formed on the element isolation insulating film, thereby forming a degraded charge film
Data Source
AI summary
A nonvolatile semiconductor memory device comprises: element isolation insulating films formed in a semiconductor substrate in a first direction; and element regions formed in a region sandwiched by the element isolation insulating film, with MONOS type memory cells. The MONOS type memory cell comprises: a tunnel insulating film disposed on the element region; a charge storage film disposed continuously on the element regions and the element isolation insulating films. The charge storage film comprises: a charge film disposed on the element region and having a certain charge trapping characteristic; and a degraded charge film disposed on the element isolation insulating film and having a charge trapping characteristic inferior to that of the charge film. The degraded charge film has a length of an upper surface thereof set shorter than a length of a lower surface thereof in a cross-section along the first direction.