GaN Gate Metal Stack With Barrier Layer for Low Leakage
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Solution Overview
Problem
The existing methods for forming electrode structures on Gallium Nitride (GaN) and Aluminum Gallium Nitride (AlGaN) substrates face challenges such as high leakage currents and inadequate coverage of metal layers on the substrate surfaces, particularly due to directional scattering and lateral diffusion during the reactive evaporation process, which can lead to incomplete coverage and exposure of the substrate.
Innovation Solution
A two-step process involving reactive evaporation for initial gate metal layer deposition followed by sputtering for additional layer coverage, using high barrier height metals like nickel (Ni) and tungsten (W) to form a gate metal structure, and employing a tungsten nitride (WN) barrier layer to prevent diffusion of low barrier height metals like aluminum (Al), ensuring complete coverage and reduced crystal damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If reactive evaporation is used for gate metal layer deposition, then the initial gate metal layer can be formed, but directional scattering and lateral diffusion cause incomplete coverage and exposure of the substrate
Solution Approach 1:
The metal layer formation is divided into two separate steps: first reactive evaporation to deposit the initial gate metal layer, then sputtering to deposit an additional metal layer. This segmentation allows each process to contribute its strengths - reactive evaporation for initial coverage and sputtering for complete coverage including sidewalls and corners that directional scattering misses.
Solution Approach 2:
The reactive evaporation process is performed first as a preliminary action to deposit the initial gate metal layer on the substrate. This preliminary layer serves as a foundation that is then enhanced by the subsequent sputtering process, which adds material to ensure complete coverage of all surfaces including vertical sidewalls and corners that the preliminary reactive evaporation could not fully reach.
2Reliability
If low barrier height metals like aluminum are used, then high conductivity is achieved, but metal diffusion occurs compromising device performance
Solution Approach 1:
A barrier metal layer is introduced as an intermediary between the low barrier height metal (aluminum) and the substrate. This intermediary layer prevents direct diffusion of aluminum into the substrate while allowing the aluminum to maintain its high conductivity function. The barrier layer acts as a mediator that blocks harmful diffusion pathways while preserving the electrical performance benefits of the aluminum gate metal.
Solution Approach 2:
The gate structure uses a composite material approach by combining multiple metal layers with different properties. The barrier metal layer provides diffusion protection while the aluminum layer provides high conductivity. This composite structure leverages the complementary strengths of different materials to simultaneously achieve both device performance and protection against harmful diffusion effects.
3Ease of manufacture
If single-step reactive evaporation is used, then the process is simple, but inadequate coverage occurs on substrate surfaces
Solution Approach 1:
The manufacturing process is segmented into two distinct deposition steps: reactive evaporation followed by sputtering. While this increases process complexity compared to a single step, each segment is optimized for its specific function - reactive evaporation for initial layer formation and sputtering for complete surface coverage including difficult-to-reach areas. The segmentation trades some process simplicity for significantly improved coverage completeness.
Solution Approach 2:
The deposition parameters are changed between the two steps - reactive evaporation uses directional vapor deposition parameters, while sputtering uses plasma-based parameters that enable conformal coverage. By changing the deposition method and its associated parameters, the process achieves complete coverage of substrate surfaces including vertical sidewalls and corners that a single reactive evaporation step cannot cover.
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 results in improved barrier height and reduced leakage currents, enhancing the performance and reliability of GaN and AlGaN high electron mobility transistor (HEMT) devices by ensuring complete coverage and preventing metal diffusion, thus maintaining high conductivity and stability.
Implementation Method 1
reactive evaporation for initial gate metal layer deposition
Implementation Method 2
sputtering for additional layer coverage
Implementation Method 3
employing a tungsten nitride (WN) barrier layer to prevent diffusion of low barrier height metals like aluminum (Al)
Data Source
AI summary
Electrode structures and methods of manufacturing electrode structures for devices are described. An example electrode structure includes a gate metal formation including a nitride layer with an opening that exposes a surface region of a substrate, a gate metal layer on the surface region of the substrate, a barrier metal layer on the gate metal layer and on at least a portion of a step around the opening in the nitride layer, and a conductive metal layer on the barrier metal layer. The gate metal layer is on the surface region of the substrate and on at least another portion of the step around the opening in the nitride layer in one example. The gate metal layer includes first and second gate metal layers in one example, such as nickel and tungsten.