Two-Stage Plasma Etching for Enhancement Mode GaN HFET
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Solution Overview
Problem
Existing methods for converting depletion mode AlGaN/GaN HFET/HEMT devices to enhancement mode (normally-off operation) face challenges such as non-uniformity, material damage, and instability, particularly with fluorine implantation and traditional gate recess etching, which affect the uniformity and reliability of threshold voltages across wafers.
Innovation Solution
A two-step plasma etching process using an oxygen plasma to oxidize the AlGaN/GaN surface followed by a Boron tricloride plasma to selectively remove material, creating a self-limiting etching process that thins the AlGaN layer under the gate, thereby shifting the conduction band above the Fermi energy without causing deep damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If fluorine implantation is used to deplete 2DEG under the gate, then the device becomes normally-off, but the uniformity of threshold voltage across the wafer deteriorates due to non-uniform plasma distribution
Solution Approach 1:
The patent changes the chemical parameters of the plasma etching process by using a specific gas mixture (CF4 and O2) and controlling the ratio of fluorine to oxygen. This parameter optimization enables uniform threshold voltage across the wafer while maintaining normally-off operation, resolving the contradiction between device reliability and manufacturing precision.
2Reliability
If traditional gate recess etching is used to thin the AlGaN barrier layer, then the conduction band shifts above Fermi energy creating normally-off device, but deep material damage occurs reducing device performance
Solution Approach 1:
The patent introduces oxygen into the plasma etching process, which oxidizes the AlGaN surface during etching. This oxidation protects the underlying GaN channel from plasma damage while the fluorine continues to deplete the 2DEG under the gate, achieving normally-off operation without deep material damage.
3Reliability
If the AlGaN barrier layer is grown thinner to eliminate 2DEG, then the device becomes normally-off, but the low-resistance high-mobility conduction path between source and drain is lost
Solution Approach 1:
The patent applies local quality by creating spatially non-uniform fluorine distribution through plasma etching. The fluorine concentration is highest under the gate where it depletes the 2DEG to create normally-off operation, while the source and drain regions maintain sufficient 2DEG concentration for low-resistance conduction paths.
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 achieves improved control and uniformity in etching, reducing material damage and enhancing the reliability and performance of GaN enhancement mode HFETs/HEMTs by maintaining the beneficial properties of the 2DEG across the entire wafer, resulting in lower leakage currents and more uniform threshold voltages.
Implementation Method 1
In the first step the AlGaN is oxidized with an O2 plasma
Implementation Method 2
In the second step, the Gate area in the AlGaN is etched with a Boron tricloride plasma
Data Source
AI summary
A two stage plasma etching technique is described that allows the fabrication of an enhancement mode GaN HFET/HEMT. A gate recess area is formed in the Aluminum Gallium Nitride barrier layer of an GaN HFET/HEMT. The gate recess is formed by a two stage etching process. The first stage of the technique uses oxygen to oxidize the surface of the Aluminum Gallium Nitride barrier layer below the gate. Then the second stage uses Boron tricloride to remove the oxidized layer. The result is a self limiting etch process that uniformly thins the Aluminum Gallium Nitride layer below the HFET's gate region such that the two dimensional electron gas is not formed below the gate, thus creating an enhancement mode HFET.