Metal Gate Work Function Control via Thickness Variation
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Solution Overview
Problem
The miniaturization of MISFETs requires a thin gate insulating film, leading to depletion issues with polycrystalline silicon gate electrodes, and metal gate electrodes lack the ability to adjust work function through impurity introduction, affecting threshold voltage and carrier mobility.
Innovation Solution
The use of metal gate electrodes with varying thicknesses to differentiate work functions for n-type and p-type MISFETs, combined with impurity implantation into gate insulating films to adjust threshold voltage, while maintaining carrier mobility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a metal gate electrode is used to avoid depletion issues with thin gate insulating films, then reliability is improved, but the ability to adjust work function through impurity introduction is lost, affecting threshold voltage control
Solution Approach 1:
The patent changes the physical parameter of the gate electrode from polycrystalline silicon to metal, which fundamentally alters the approach to work function adjustment. Instead of using impurity concentration changes in silicon, the invention uses the inherent work function properties of different metal materials and their thickness variations to achieve the desired threshold voltage control for both n-type and p-type MISFETs.
Solution Approach 2:
The invention employs composite gate electrode structures consisting of multiple metal layers with different thicknesses and materials. By combining metals such as tungsten, titanium nitride, and tantalum nitride in specific configurations, the system achieves both depletion resistance and adjustable work function characteristics that neither material could provide alone.
2Adaptability or versatility
If impurity implantation is used to adjust threshold voltage in metal gate electrodes, then threshold voltage control is improved, but carrier mobility decreases due to impurity damage
Solution Approach 1:
The invention extracts the impurity implantation step from the threshold voltage adjustment process. Instead of implanting impurities into the metal gate electrode or channel region, the patent adjusts threshold voltage by controlling the thickness and work function of the metal gate layers themselves, thereby eliminating the harmful effects of impurity implantation on carrier mobility.
Solution Approach 2:
The metal gate electrode serves as an intermediary that mediates between the gate insulating film and the channel region. By adjusting the metal layer thickness and material composition, the system achieves threshold voltage control without requiring impurity introduction into the sensitive channel region, thus preserving carrier mobility.
3Productivity
If the gate insulating film is thinned to enable MISFET miniaturization, then productivity is improved, but depletion of polycrystalline silicon gate electrodes occurs
Solution Approach 1:
The invention replaces the polycrystalline silicon gate electrode with a metal gate electrode structure that is more resistant to depletion effects. The metal layers, particularly tungsten and its nitrides, provide superior electrical stability and depletion resistance compared to thin polycrystalline silicon, enabling reliable operation with thinned gate insulating films.
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 allows for suitable threshold voltage setting and reduced deterioration in operating characteristics by differentiating film thickness and using impurity implantation, effectively addressing the limitations of metal gate electrodes.
Implementation Method 1
The second metal layer 22a is thicker than the first metal layer 12a and has the same constituent element as the first metal layer. By differentiating film thickness, the patent adjusts the work function of the metal gate electrode to achieve suitable threshold voltage for both n-type and p-type MISFETs.
Implementation Method 2
As a method of adjusting threshold voltage of a MISFET using a metal gate electrode, a technique is known in which an impurity such as La, etc., is implanted into a gate insulating film
Implementation Method 3
a technique is known in which a metal gate electrode is formed so as to include a stress for generating a strain in a channel region of a MISFET, thereby modulating carrier mobility
Implementation Method 4
it is possible to include a tensile stress by forming a metal gate electrode using a CVD (Chemical Vapor Deposition) method
Implementation Method 5
to include a compressive stress by forming a metal gate electrode using a PVD (Physical Vapor Deposition) method
Data Source
AI summary
A semiconductor device according to one embodiment includes: an n-type transistor comprising a first gate electrode formed on a semiconductor substrate via a first gate insulating film, a first channel region formed in the semiconductor substrate under the first gate insulating film, and first source/drain regions formed in the semiconductor substrate on both sides of the first channel region, the first gate electrode comprising a first metal layer and a first conductive layer thereon; and a p-type transistor comprising a second gate electrode formed on the semiconductor substrate via a second gate insulating film, a second channel region formed in the semiconductor substrate under the second gate insulating film, and second source/drain regions formed in the semiconductor substrate on both sides of the second channel region, the second gate electrode comprising a second metal layer and a second conductive layer thereon, the second metal layer being thicker than the first metal layer and having the same constituent element as the first metal layer.