Isolating Trench Encapsulation for Gate Work Function Control
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Solution Overview
Problem
In the production of MOS transistors with high-permittivity dielectric gates, the diffusion of lanthanum or magnesium during thermal stages leads to uncontrolled reduction in their concentration, affecting the gate's work function and thereby modifying the transistor's operating characteristics.
Innovation Solution
The integration of an insulating encapsulation layer comprising nitrogen or carbon around the isolating trench, which acts as a shield to slow down the diffusion of lanthanum or magnesium, is implemented, along with the use of silicon nitride spacers and a SiOxN1-x interface layer to reduce absorption by oxygen reservoirs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If thermal stages are used in the production process, then the transistor can be manufactured with high-permittivity dielectric gate, but the lanthanum or magnesium diffuses and their concentration reduces uncontrollably
Solution Approach 1:
A nitrogen-containing layer is introduced as an intermediary barrier between the lanthanum/magnesium layer and the oxygen reservoirs in the isolating trench. This intermediate layer prevents direct interaction between the diffusing metal atoms and oxygen, blocking the formation of metal oxides and maintaining the original metal concentration in the gate structure.
Solution Approach 2:
The oxygen reservoir function of the isolating trench is extracted and neutralized by filling it with a nitrogen-containing material. This removes the harmful oxygen supply that causes uncontrolled diffusion and oxide formation, while the trench structure itself is retained for isolation purposes.
2Ease of manufacture
If the isolating trench is used to delimit the active area, then the transistor structure is defined, but the trench creates oxygen reservoirs that absorb lanthanum or magnesium
Solution Approach 1:
The oxygen reservoir property of the isolating trench, which was previously harmful, is converted into a beneficial feature by replacing the oxygen-containing material with a nitrogen-containing material. The trench continues to provide mechanical definition and isolation, but now acts as a diffusion barrier rather than an oxygen source, preventing metal oxide formation.
3Manufacturing precision
If lanthanum or magnesium layer is deposited to adjust gate work function, then the desired electrical characteristics are achieved, but subsequent thermal processing causes uncontrolled diffusion
Solution Approach 1:
The nitrogen-containing layer is deposited beforehand, before the lanthanum or magnesium layer, to create a protective barrier in advance. This preliminary action ensures that when subsequent thermal processing occurs, the metal atoms already have a diffusion barrier in place, preventing the uncontrolled concentration reduction that would otherwise occur.
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 solution effectively controls the gate work function, maintaining the transistor's operating characteristics by limiting the diffusion of lanthanum or magnesium, thereby ensuring consistent threshold voltage and improved transistor performance.
Implementation Method 1
The lanthanum present in this layer 14 tends to diffuse during each of the subsequent stages that requires a certain thermal budget
Implementation Method 2
The lanthanum has a tendency to be absorbed by the oxygen reservoirs and to form compounds of the La2O3 or lanthanum silicate type
Data Source
AI summary
An integrated circuit including at least one isolating trench that delimits an active area made of a monocrystalline semiconductor material, the or each trench comprising an upper portion including an insulating layer that encapsulates a lower portion of the trench, the lower portion being at least partly buried in the active area and the encapsulation layer comprising nitrogen or carbon.