GaN Gate Metal Stack With Step Coverage and Leakage Control
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Solution Overview
Problem
Existing methods for forming gate metal structures on Gallium Nitride (GaN) and Aluminum Gallium Nitride (AlGaN) substrates face challenges such as inadequate coverage of metal layers over steps in the silicon nitride layer, leading to potential exposure of the substrate and high leakage currents due to directional scattering and lateral diffusion during reactive evaporation processes.
Innovation Solution
A two-step process involving reactive evaporation for depositing gate metal layers, followed by sputtering for additional metal layers to cover discontinuities and prevent substrate damage, using high barrier height metals like nickel and tungsten, and incorporating a tungsten nitride barrier layer to prevent diffusion of conductive metals like aluminum.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If reactive evaporation is used to deposit gate metal layers, then the metal layers can be deposited with controlled thickness and composition, but the directional scattering causes inadequate coverage over steps in the silicon nitride layer
Solution Approach 1:
The patent divides the metal deposition process into multiple sequential steps, using different deposition techniques (reactive evaporation followed by sputtering) to address different coverage requirements. The first step deposits initial metal layers with controlled thickness, while subsequent steps fill in coverage gaps over steps and discontinuities in the silicon nitride layer.
Solution Approach 2:
The patent transitions from a single-planar deposition approach to a multi-dimensional solution by using sputtering to deposit metal layers that conformally cover three-dimensional features including steps and discontinuities in the silicon nitride layer, ensuring complete coverage from multiple angular perspectives.
2Manufacturing precision
If reactive evaporation is used for metal layer deposition, then deposition control is improved, but lateral diffusion occurs leading to high leakage currents
Solution Approach 1:
The patent introduces a barrier metal layer (such as tungsten nitride) as an intermediary between the gate metal layer and the underlying structures. This barrier layer prevents lateral diffusion of conductive metals into the silicon nitride layer and substrate, thereby eliminating leakage current paths while allowing the gate metal to maintain its controlled deposition characteristics.
Solution Approach 2:
The patent creates a composite metal stack structure combining different metal layers (gate metal, barrier metal, and additional metal layers) with distinct functional properties. The barrier metal layer specifically addresses the leakage current issue by providing a diffusion barrier, while the other layers maintain electrical conductivity and gate functionality.
3Reliability
If additional metal layers are deposited to cover discontinuities, then substrate protection is improved, but the process complexity increases
Solution Approach 1:
The patent designs the additional metal layers and barrier layers to serve multiple functions simultaneously: they provide substrate protection, fill coverage gaps over steps, prevent lateral diffusion, and maintain electrical continuity. This multi-functionality reduces the need for separate dedicated steps for each function, thereby limiting the increase in overall process complexity.
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 ensures complete coverage of the substrate, reduces leakage currents, and maintains the integrity of the silicon nitride layer, enhancing the performance and reliability of GaN and AlGaN high electron mobility transistors by minimizing crystal damage and diffusion issues.
Implementation Method 1
depositing the gate metal layer includes depositing the gate metal layer using reactive evaporation
Implementation Method 2
depositing the barrier metal layer includes sputtering tungsten nitride over the gate metal layer, a top surface of the silicon nitride layer, and at least a portion of a step around the opening in the silicon nitride layer
Data Source
AI summary
A method of manufacturing an electrode structure for a device, such as a GaN or AlGaN device is described. In one example, the method includes providing a substrate (212) of GaN or AlGaN with a surface region of the GaN or AlGaN exposed through an opening (216) in a layer of silicon nitride (214) formed on the substrate. The method further includes depositing layers of W (222), in one example, or Ni (220) and W (222), in another example, on the substrate and the layer of silicon nitride using reactive evaporation and photoresist layers (230) having an undercut profile for liftoff. The method further includes removing the photoresist layers having the undercut profile, and depositing layers of WN (224) and Al over the underlying layers of W or Ni and W by sputtering.