Scan Flip-Flop Overwrite Feature for LBIST Test Coverage
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing Logic Built-in Self-Test (LBIST) systems face challenges in identifying and correcting scan chain errors without significant design changes and cost impacts, especially at advanced process nodes, as unexpected results may not indicate functional issues, leading to reduced test coverage when scan chains are disabled.
Innovation Solution
Incorporating modifiable flip-flops with an overwrite feature that allows resetting captured unexpected values during scan shift operations, enabled by a metal mask option, to output correct values, thereby maintaining test coverage with minimal design and cost changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If scan chains are disabled to avoid costly mask layer changes, then manufacturing cost is reduced, but test coverage is significantly reduced
Solution Approach 1:
The patent segments the scan chain into individual flip-flops, allowing selective modification of only those flip-flops that capture unexpected values. This enables fixing specific problematic flip-flops without disabling entire scan chains, thereby maintaining test coverage while avoiding costly mask layer changes.
Solution Approach 2:
The patent applies local quality by making modifications only to specific flip-flops that exhibit unexpected behavior, rather than uniformly modifying or disabling entire scan chains. This targeted approach preserves test coverage for unaffected scan chains while correcting local issues.
2Reliability
If multiple mask layers are changed to fix scan chain errors, then scan chain functionality is corrected, but manufacturing cost increases significantly
Solution Approach 1:
The patent extracts the problematic flip-flops from the scan chain and applies modifications only to those specific elements. This allows correcting scan chain functionality by targeting only the defective portions rather than requiring comprehensive mask layer changes across the entire circuit.
Solution Approach 2:
The patent changes the state or configuration parameter of specific flip-flops (e.g., their output values or capture behavior) to correct unexpected results. This parameter-level modification approach enables functionality correction without requiring expensive structural changes to multiple mask layers.
3Measurement precision
If engineering change order fixes are applied to correct scan chain errors, then test accuracy is improved, but device complexity and manufacturing cost increase
Solution Approach 1:
The patent segments the correction process to apply fixes only to individual flip-flops showing unexpected behavior, rather than implementing comprehensive engineering change orders across the entire device. This reduces design complexity while maintaining test accuracy for the affected elements.
Data Source
AI summary
A scan circuit and methods of operating a scan circuit are provided. The method for operating a scan circuit includes providing a first scan flip-flop which includes an overwrite feature. With the overwrite feature enabled, a change in functional behavior of the first scan flip-flop occurs based on a control signal. The method may further include capturing data at a first input of the first scan flip-flop during a first state of the control signal and resetting captured data by using the overwrite feature during a first transition of the control signal. The method may further include forming a scan chain with one or more of the first scan flip-flops and one or more second scan flip-flops. The second scan flip-flops may include a similar overwrite feature, having the overwrite feature disabled.