Split Gate FinFET Threshold Voltage Control via Work Function Engineering
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Solution Overview
Problem
In split gate-type memory cells with fin-type channels, the threshold voltage decreases as the fin width decreases, leading to increased channel resistance and reduced mobility, which compromises the driving force of the semiconductor device.
Innovation Solution
The implementation of a semiconductor device with a control gate electrode and a memory gate electrode formed from n-type polycrystalline silicon, where a first metal film with a higher work function than the second metal film is used between the insulating films, allowing for controlled threshold voltage adjustment without increasing impurity concentration, thereby maintaining the fin-type channel's effectiveness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the fin width is decreased to increase integration density, then the integration density is improved, but the threshold voltage decreases and channel resistance increases
Solution Approach 1:
The patent applies local quality by introducing metal films with different work functions at specific locations within the gate structure. The first metal film (higher work function) and second metal film (lower work function) are positioned at different regions of the gate electrode, creating localized electrical property variations that enable independent control of threshold voltage and channel characteristics without changing the overall fin width
Solution Approach 2:
The patent changes the electrical parameters of the gate structure by selecting metal films with different work functions. By adjusting the work function parameter through material selection (e.g., tungsten for higher work function, molybdenum for lower work function) and controlling the thickness ratio between the first and second metal films, the threshold voltage can be precisely controlled while maintaining small fin dimensions
2Reliability
If the impurity concentration of the fin is increased to increase threshold voltage, then the threshold voltage is improved, but the channel resistance increases and mobility decreases
Solution Approach 1:
The patent changes the work function parameter of the gate electrode by incorporating metal films with different work functions. This allows independent adjustment of threshold voltage without modifying the impurity concentration in the fin, thereby maintaining high carrier mobility and low channel resistance while achieving the desired threshold voltage level
Solution Approach 2:
The gate electrode is constructed as a composite structure comprising a semiconductor material (polycrystalline silicon or silicon-germanium) combined with metal films having different work functions. This composite gate structure enables fine-tuned control of electrical characteristics, allowing threshold voltage adjustment through metal film composition and thickness ratio without affecting the fin's impurity concentration and carrier mobility
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach prevents degradation of operation characteristics by maintaining low S value variation and high driving force, even at smaller fin widths, while allowing for stable threshold voltage control and reduced channel resistance.
Implementation Method 1
A work function of the first metal film is greater than a work function of the second metal film
Data Source
AI summary
A memory cell includes a control gate electrode and a memory gate electrode. The control gate electrode is formed over the upper surface and the sidewall of a fin FA including apart of a semiconductor substrate. The memory gate electrode is formed over one side surface of the control gate electrode and the upper surface and the sidewall of the fin through an ONO film, in a position adjacent to the one side surface of the control gate electrode. Further, the control gate electrode and the memory gate electrode are formed of n-type polycrystalline silicon. A first metal film is provided between the gate electrode and the control gate electrode. A second metal film is provided between the ONO film and the memory gate electrode. A work function of the first metal film is greater than a work function of the second metal film.