Bent Word Line Layout for NAND Flash Contact Resistance
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Solution Overview
Problem
Current techniques for downsizing NAND cell type flash memory face challenges in forming contact holes and achieving sufficient fringe sizes for word lines, leading to increased contact resistance and potential short circuits due to limitations in lithography resolution and pitch constraints.
Innovation Solution
A word line layout is proposed where one end of the word line is bent towards the select gate line, with a contact plug connected between the bent point and the distal end, allowing for adjustable positions, sizes, and pitches in a two-dimensional manner, thereby avoiding contact resistance and short-circuit issues.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the pitch of word lines is reduced to downsize memory cells, then memory capacity increases, but contact resistance increases and short circuits occur due to insufficient fringe size
Solution Approach 1:
The word line layout transitions from a conventional straight configuration to a bent configuration that extends into the second dimension (towards the select gate line side). This dimensional change creates additional space for contact plugs without increasing the linear pitch between word lines, thereby maintaining memory capacity while improving contact reliability
Solution Approach 2:
The word line is designed with a curved or bent shape rather than a straight line. This curvature allows the word line to create a larger effective contact area and provides geometric clearance from adjacent word lines, preventing short circuits while maintaining electrical connectivity through the contact plug
2Productivity
If the pitch of word lines is reduced to downsize memory cells, then memory capacity increases, but manufacturing precision decreases due to lithography resolution limits
Solution Approach 1:
By utilizing the second dimension (space towards the select gate line) for the bent portion of the word line, the design decouples the contact area from the linear pitch constraints. This allows contact plugs to be positioned with sufficient manufacturing precision even when word line pitches are reduced for higher capacity
Solution Approach 2:
The word line layout employs asymmetric design where one end of the word line is bent towards the select gate line while the other end remains straight. This asymmetric configuration optimizes the contact area placement without requiring symmetric reduction of all dimensions, making the design more tolerant to lithography variations
3Area of stationary object
If fringes are directly connected to both ends of word lines in a zigzag manner, then contact area is allocated, but fringe size is insufficient and alignment shifts cause short circuits
Solution Approach 1:
The contact plug is positioned in the second dimension (towards the select gate line side) rather than directly at the ends of the word line. This spatial repositioning creates sufficient clearance from adjacent word lines, preventing short circuits while maintaining adequate contact area for reliable electrical connection
Solution Approach 2:
The bent portion of the word line acts as an intermediary structure that provides geometric clearance between the contact plug and adjacent word lines. This intermediate configuration prevents direct alignment issues that would cause short circuits while maintaining electrical connectivity
Data Source
AI summary
A nonvolatile semiconductor memory includes a cell unit having a select gate transistor and a memory cell connected in series, a select gate line connected to the select gate transistor, and a word line connected to the memory cell. One end of the word line is bent to the select gate line side, and a fringe is connected between a bent point and a distal end of the word line.