Multi-layer work function metal gates for vFET threshold voltage control
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Solution Overview
Problem
There is a challenge in maintaining performance improvements in deep submicron semiconductor devices without scaling down dimensions, particularly in achieving precise threshold voltage control for high-performance small field effect transistors (FETs) using high-k materials, where every element influences the threshold voltage and variation becomes less tolerable.
Innovation Solution
The method involves forming multiple field effect transistors (FETs) with different work function gate stacks, using a combination of high-k dielectric layers and various work function conducting layers to achieve distinct threshold voltages, allowing for similar gate thickness across FETs while fine-tuning the thickness difference to less than 3 nm, enabling precise control of threshold voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If high-k materials are used to increase gate capacitance without scaling down dimensions, then gate capacitance is improved, but threshold voltage control precision deteriorates
Solution Approach 1:
The gate stack is segmented into multiple functional layers: a high-k dielectric layer for capacitance enhancement and separate work function conducting layers for threshold voltage control. This segmentation allows independent optimization of each layer's function, resolving the contradiction between achieving high gate capacitance and maintaining precise threshold voltage control.
Solution Approach 2:
The gate structure uses composite materials combining high-k dielectric materials (for high capacitance) with work function conducting layers (for precise threshold control). This composite approach allows the structure to simultaneously achieve high gate capacitance from the dielectric and precise threshold voltage control from the conducting layers, directly resolving the technical contradiction.
2Manufacturing precision
If different gate materials are used to tune threshold voltage, then threshold voltage control is improved, but device complexity increases
Solution Approach 1:
Different work function conducting layers are applied locally to specific FET regions depending on the desired threshold voltage characteristics. This local quality approach allows threshold voltage tuning without requiring completely different gate stacks for each FET, thereby reducing overall device complexity while maintaining precise threshold control.
Solution Approach 2:
The invention tunes threshold voltage by changing the work function parameters of the conducting layers rather than changing the fundamental gate stack structure. This parameter-based tuning approach maintains structural consistency across devices, reducing complexity compared to using entirely different gate materials for each threshold voltage requirement.
3Manufacturing precision
If gate thickness is reduced to achieve different threshold voltages, then threshold voltage differentiation is improved, but gate capacitance decreases
Solution Approach 1:
Instead of differentiating threshold voltages through gate thickness variations (one dimension), the invention uses work function conducting layers with different material compositions (another dimension - material property). This allows threshold voltage differentiation while maintaining uniform gate thickness and high gate capacitance from the high-k dielectric layer.
Solution Approach 2:
The gate structure combines high-k dielectric material for capacitance with work function conducting layers for threshold differentiation. This composite approach enables threshold voltage differentiation through material selection rather than thickness variation, preserving gate capacitance while achieving the desired threshold voltage differentiation.
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 the achievement of multiple threshold voltages on fully depleted channel architectures, enhancing performance and reducing device variability by using multi-layer gate stacks with shared and distinct layers, resulting in better mobility and reduced doping requirements.
Implementation Method 1
The dielectric constant of such materials is higher than that of silicon dioxide (SiO2). A high-k material can physically be thicker than an oxide and still have a lower equivalent oxide thickness (EOT) value.
Implementation Method 2
Techniques exist to tune device thresholds through the modification of the gate work function.
Data Source
AI summary
A method is presented for forming a device having multiple field effect transistors (FETs) with each FET having a different work function. In particular, the method includes forming multiple microchips in which each FET has a different threshold voltage (Vt) or work-function. In one embodiment, four FETs are formed over a semiconductor substrate. Each FET has a source, drain and a gate electrode. Each gate electrode is processed independently to provide a substantially different threshold voltage.