Non-etch Gas Cooled GaN Epitaxial Stack Fabrication
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Solution Overview
Problem
Conventional NH3 and H2 gas mixtures during cooldown after epitaxial Group IIIA-N cap layer depositions in GaN FETs result in pits in the cap layer, leading to defects and potential device failures due to H2 attacking the cap layers, causing increased roughness and pit density.
Innovation Solution
A cooling process using a gas mixture of NH3 and at least one other gas, such as N2, Ar, He, or Ne, is employed to create a non-etching ambient in the deposition chamber, reducing pit density and surface roughness to less than 10 pits per square μm with average diameters less than 0.05 μm, thereby preventing cap layer defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If conventional NH3 and H2 gas mixture is used during cooldown after cap layer deposition, then cooling efficiency is improved, but cap layer surface quality deteriorates due to H2 attacking the cap layer and forming pits
Solution Approach 1:
The patent changes the gas composition parameters during cooldown by replacing H2 with inert gases (N2, Ar, He, or Ne) in the gas mixture. This parameter change eliminates the chemical attack on the cap layer while maintaining efficient cooling, thereby resolving the contradiction between cooling efficiency and surface quality.
Solution Approach 2:
The patent introduces an inert atmosphere using gases such as N2, Ar, He, or Ne during the cooldown process. This inert environment prevents H2 from attacking the cap layer, avoiding pit formation while still allowing effective cooling to occur.
2Speed
If H2 is used in the gas mixture during cooldown, then cooling rate is improved, but defect density increases due to preferential etching of pits
Solution Approach 1:
The patent modifies the gas mixture composition by eliminating H2 and substituting it with inert gases. This parameter change maintains the cooling rate while preventing the chemical reactions that cause pit formation and subsequent defect density increase.
Solution Approach 2:
The patent converts the potentially harmful effect of rapid cooling (which can cause thermal stress) into a benefit by using inert gases that enable fast cooling without the side effect of chemical attack on the cap layer, thus achieving high cooling rates without increased defect density.
3Ease of manufacture
If conventional cooling process with H2 is used, then process simplicity is maintained, but device reliability deteriorates due to cap layer pits and defects
Solution Approach 1:
The patent changes the gas composition parameter from H2-containing mixture to inert gas-containing mixture. This simple parameter change maintains process simplicity while dramatically improving device reliability by eliminating cap layer pits and associated defects.
Solution Approach 2:
The patent employs an inert atmosphere during cooldown, which is a simple process modification that prevents cap layer degradation. This approach maintains ease of manufacture while significantly improving device reliability by avoiding pit formation.
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 higher transistor breakdown voltage, lower leakage current, and reduced substrate bow/warp, with power transistors achieving a breakdown voltage of at least 100V at a leakage current density of 1 μA/mm² and significantly reducing device failure rates.
Implementation Method 1
H2 can attack Group IIIA-N cap layers such as GaN or AlGaN causing pits
Implementation Method 2
epitaxial Group IIIA-N cap layer depositions
Implementation Method 3
Conventional buffer layer and cap layer deposition processes utilize NH3 and H2 during the cool down from their respective deposition temperatures
Data Source
AI summary
A method of fabricating an epitaxial stack for Group IIIA-N transistors includes depositing at least one Group IIIA-N buffer layer on a substrate in a deposition chamber of a deposition system. At least one Group IIIA-N cap layer is then deposited on the first Group IIIA-N buffer layer. During a cool down from the deposition temperature for the cap layer deposition the gas mixture supplied to the deposition chamber includes NH3 and at least one other gas, wherein the gas mixture provide an ambient in the deposition chamber that is non-etching with respect to the cap layer so that at a surface of the cap layer there is (i) a root mean square (rms) roughness of <10 Å and (ii) a pit density for pits greater than (>) 2 nm deep less than (<) 10 pits per square μm with an average pit diameter less than (<) 0.05 μm.