3D Vertical NOR Memory Arrays With Two-Step Word Line Etching
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Solution Overview
Problem
Existing methods for forming 3-dimensional vertical NOR-type memory string arrays face challenges such as ribboning and alignment issues during the etching of local word lines, which affect the structural stability and electrical resistance of the word line stacks.
Innovation Solution
A method that involves etching local word lines in two steps, reducing the aspect ratio and using damascene local vertical bit lines to improve structural stability, and employing a staircase structure for contacting local word lines to address alignment issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If local word lines are etched in one step, then the manufacturing process is simpler, but the aspect ratio is high causing structural instability and ribboning
Solution Approach 1:
The etching of local word lines is divided into two separate steps: first etching through the first plurality of word lines, then etching through the second plurality of word lines. This segmentation reduces the aspect ratio of each etching step, preventing structural instability and ribboning that would occur in a single high aspect ratio etching step.
2Stability of the object's composition
If local word lines are etched in two steps, then the aspect ratio is reduced improving structural stability, but alignment issues occur between the two etching steps
Solution Approach 1:
A mandrel structure is used as an intermediary element to maintain alignment between the two etching steps. The mandrel is positioned between the first and second pluralities of local word lines and serves as a reference structure that ensures proper alignment during the sequential etching process, preventing misalignment issues.
3Reliability
If conventional etching methods are used, then ribboning occurs affecting electrical resistance, but modifying the etching process increases process complexity
Solution Approach 1:
The etching process is segmented into two steps with a mandrel structure in between, which prevents ribboning by reducing the aspect ratio of each etching step. This segmentation ensures consistent electrical resistance by maintaining uniform word line widths throughout the structure.
Solution Approach 2:
The mandrel structure acts as an intermediary that maintains alignment and prevents ribboning during the two-step etching process. By providing a physical reference structure, it ensures that the word lines are etched with consistent dimensions, thereby maintaining reliable electrical resistance characteristics.
Data Source
AI summary
A method for forming 3-dimensional vertical NOR-type memory string arrays uses damascene local bit lines is provided. The method of the present invention also avoids ribboning by etching local word lines in two steps. By etching the local word lines in two steps, the aspect ratio in the patterning and etching of stack of local word lines (“word line stacks”) is reduced, which improves the structural stability of the word line stacks.