Stacked Transistors With Backside Work Function Fill for Threshold Control
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Solution Overview
Problem
Existing semiconductor fabrication methods for stacked transistors face challenges in maintaining super-low threshold voltages for lower transistors due to high temperatures during the formation of upper transistors, leading to shifts in voltage levels.
Innovation Solution
The method involves filling the work function material for lower transistors from the backside of the semiconductor structure, avoiding exposure to high temperatures during upper transistor formation, thereby maintaining different threshold voltages, including super-low threshold voltages, by using a second work function material that extends below the first work function material.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If upper transistors are formed using conventional high-temperature processes, then upper transistor performance is improved, but lower transistor threshold voltage shifts occur
Solution Approach 1:
The fabrication process is divided into separate sequences: frontside processing for upper transistors and backside processing for lower transistors. This segmentation allows each transistor type to undergo appropriate thermal processing independently, preventing threshold voltage shifts in lower transistors while maintaining upper transistor performance
Solution Approach 2:
The conventional fabrication sequence is inverted by forming lower transistors first on the backside of the substrate, then forming upper transistors on the frontside. This reversal ensures that lower transistors are established before subsequent high-temperature processing, protecting their threshold voltage characteristics
2Ease of manufacture
If work function material is filled from the frontside only, then process simplicity is maintained, but super-low threshold voltage cannot be achieved for lower transistors
Solution Approach 1:
The work function material filling process extends from a single dimension (frontside only) to multiple dimensions by incorporating backside filling. This dimensional expansion enables precise control of lower transistor threshold voltages while maintaining compatibility with existing frontside processing for upper transistors
Data Source
AI summary
Embodiments include a semiconductor structure having a first lower transistor and a second lower transistor. Upper transistors are formed above the first and second lower transistors, the first lower transistor having a first work function material, the second lower transistor having a second work function material different from the first work function material. A portion of the second work function material extends below a bottom surface of the first work function material.