SRAM Metal Gate Work Function Segmentation
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Solution Overview
Problem
The challenge in semiconductor manufacturing lies in the complexity and uncertainty of removing p-type work function material in NMOS transistors during the fabrication of metal gate CMOS transistors, especially at scaled-down processes like 22 nm or less, which affects process control and leads to non-uniform dopant distribution and threshold voltage fluctuations.
Innovation Solution
Implementing a gate-last process flow that includes forming recesses for metal gate electrodes, using a protection layer to manage the removal of p-type work function material, and adjusting work function metallic layers to ensure precise control and uniformity, particularly in SRAM devices with different work function materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If p-type work function material is removed in NMOS transistors during metal gate fabrication, then metal gate CMOS transistors can be formed with proper work function, but the removal process becomes complex and uncertain at scaled-down processes like 22 nm or less, leading to non-uniform dopant distribution and threshold voltage fluctuations
Solution Approach 1:
The patent segments the gate structure into distinct components: a metal gate electrode layer and separate work function metallic layers (p-type and n-type). This segmentation allows independent formation and control of each layer, simplifying the overall fabrication process while ensuring proper work function adjustment for both PMOS and NMOS transistors without complex removal steps
Solution Approach 2:
The patent applies local quality by using different work function metallic layers in different regions: p-type work function material is applied to PMOS transistor gates, while n-type work function material is applied to NMOS transistor gates. This localized approach ensures each transistor type receives the appropriate work function adjustment without affecting the other, eliminating the need for complex selective removal processes
2Productivity
If feature sizes are scaled down to increase functional density, then production efficiency increases and costs decrease, but power dissipation increases and process control becomes more difficult
Solution Approach 1:
The patent performs preliminary action by forming the work function metallic layers before final metal gate electrode deposition. This sequence allows the work function to be established early in the process, enabling better control over transistor threshold voltages even as feature sizes scale down, while the subsequent metal gate formation completes the structure without disrupting the already-established work function characteristics
Data Source
AI summary
A SRAM device with metal gate transistors is provided. The SRAM device includes a PMOS structure and an NMOS structure over a substrate. Each of the PMOS and the NMOS structure includes a p-type metallic work function layer and an n-type metallic work function layer. The p-type work metallic function layer and the n-type metallic work function layer form a combined work function for the PMOS and the NMOS structures.