Intra-cavity Nitrogen Gettering in SiC Crystal Growth
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Solution Overview
Problem
Graphite components in SiC crystal growth chambers contaminate the environment with nitrogen, leading to undesirable nitrogen levels in grown SiC crystals, which affect the resistivity of wafers and make them less useful for microwave devices.
Innovation Solution
An in-situ gettering process using graphite to absorb nitrogen, involving degassing, activation, and nitrogen gettering steps within the crystal growth chamber, where graphite components are heated and exposed to inert gases to remove residual nitrogen before crystal growth, ensuring lower nitrogen concentrations in the growing crystals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If graphite components are used in the crystal growth chamber, then the chamber can be effectively insulated and heated, but nitrogen is absorbed and desorbed from the graphite, contaminating the crystal growth environment
Solution Approach 1:
The graphite components are pre-treated by heating to high temperature (1200-1500°C) under vacuum conditions before crystal growth to remove absorbed nitrogen and other volatile contaminants. This preliminary degassing action eliminates the harmful nitrogen that would otherwise be released during crystal growth, resolving the contradiction between using graphite for thermal insulation and avoiding nitrogen contamination.
2Quantity of substance
If high vacuum and external getters are used to remove nitrogen, then residual nitrogen content is reduced, but nitrogen is still desorbed from graphite components during crystal growth, especially in first-to-grow portions
Solution Approach 1:
The graphite components, which were previously harmful sources of nitrogen contamination, are converted into beneficial nitrogen getters by activating them through high-temperature treatment in an inert gas atmosphere. The activated graphite then preferentially absorbs nitrogen from the crystal growth environment, transforming from a harmful nitrogen source into a beneficial nitrogen removal mechanism that operates continuously during crystal growth.
Solution Approach 2:
The physical and chemical properties of graphite components are changed through high-temperature activation treatment (1200-1500°C) in inert gas atmosphere, which modifies the graphite surface structure and enhances its nitrogen absorption capacity. This parameter change transforms the graphite from a nitrogen-release material to a nitrogen-absorbing getter material.
3Manufacturing precision
If nitrogen concentration is reduced to achieve high resistivity, then wafer quality improves, but the first-to-grow portions of crystal still contain excessive nitrogen from graphite desorption
Solution Approach 1:
The graphite components are pre-activated and degassed before crystal growth begins, ensuring they are in a low-nitrogen state when growth starts. This preliminary preparation prevents excessive nitrogen release during the critical early growth stages, achieving uniform nitrogen distribution throughout the entire crystal rather than just in later portions.
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 process effectively reduces nitrogen levels in SiC crystals, increasing the yield of semi-insulating wafers with resistivity above 10^5 ohm-cm, enhancing productivity and reducing costs by minimizing nitrogen contamination during crystal growth.
Implementation Method 1
graphite is the principle source of nitrogen contamination in the growth of SiC crystals... graphite components, including the susceptor, the crucible and heat shields made of dense graphite... due to its porosity, graphite is generally capable of absorbing large quantities of gas
Implementation Method 2
The undesirable level of nitrogen in a SiC crystal is due to the desorption of nitrogen from graphite components—a process that occurs at the temperatures of SiC crystal growth
Implementation Method 3
SiC crystals are grown by a physical vapor transport (PVT) technique by sublimation of SiC and deposition of the resultant vapor on a single crystal seed of SiC
Implementation Method 4
SiC crystals are grown by a physical vapor transport (PVT) technique by sublimation of SiC and deposition of the resultant vapor on a single crystal seed of SiC
Data Source
AI summary
In method of crystal growth, an interior of a crystal growth chamber (2) is heated to a first temperature in the presence of a first vacuum pressure whereupon at least one gas absorbed in a material (4) disposed inside the chamber is degassed therefrom. The interior of the chamber is then exposed to an inert gas at a second, higher temperature in the presence of a second vacuum pressure that is at a higher pressure than the first vacuum pressure. The inert gas pressure in the chamber is then reduced to a third vacuum pressure that is between the first and second vacuum pressures and the temperature inside the chamber is lowered to a third temperature that is between the first and second temperatures, whereupon source material (10) inside the chamber vaporizes and deposits on a seed crystal (12) inside the chamber.