High-Purity Lanthanum Purification via Molten Salt Electrolysis
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Solution Overview
Problem
The production of high-purity lanthanum is hindered by its susceptibility to oxidation, leading to defects in sputtering targets and metal gate films, and the difficulty in removing rare earth impurities and other contaminants, which affects the purity and performance of semiconductor devices.
Innovation Solution
A method involving molten salt electrolysis using potassium chloride, lithium chloride, and lanthanum chloride, followed by desalting and electron beam melting, to achieve a purity of 5N or more, excluding rare earth elements and gas components, while reducing aluminum, iron, copper, and other impurities to 1 wtppm or less, and oxygen concentration to 500 wtppm or less, resulting in a stable high-purity lanthanum sputtering target and metal gate film.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional purification methods are used to produce lanthanum, then production cost and time are reduced, but the purity of lanthanum cannot reach 5N or more due to difficulty in removing rare earth impurities and other contaminants
Solution Approach 1:
The purification process is divided into multiple sequential steps: molten salt electrolysis to obtain crude lanthanum, followed by desalting treatment, then electron beam melting, and finally skull melting with slow cooling. Each step targets specific impurities and progressively increases purity to 5N or more.
Solution Approach 2:
Molten salt electrolysis uses molten salt as an intermediary medium to separate lanthanum from rare earth impurities through selective electro deposition. The desalting process uses vacuum heating as an intermediary method to remove salt contaminants, achieving the required purity level.
2Ease of operation
If lanthanum is exposed to air during handling, then processing and manufacturing become easier, but oxidation occurs rapidly causing blackening and reducing electric conductivity
Solution Approach 1:
The patent maintains lanthanum in an inert atmosphere throughout the process: electron beam melting and skull melting are performed under vacuum conditions, and the final product is stored in a sealed container filled with inert gas. This prevents oxidation while allowing complete processing and handling.
Solution Approach 2:
The inert atmosphere protection is established before any handling operations begin. The sealed container with inert gas is prepared in advance, and lanthanum is transferred into this protective environment before exposure to air, preventing oxidation during subsequent handling and storage.
3Adaptability or versatility
If lanthanum target is left exposed to air for storage, then accessibility and ease of use are improved, but moisture reaction occurs forming white hydroxide powder that makes sputtering impossible
Solution Approach 1:
The lanthanum target is stored in a sealed container filled with inert gas, creating a protective atmosphere that prevents moisture reaction. This maintains the target's sputtering functionality while allowing long-term storage and on-demand usage.
Solution Approach 2:
The sealed container with inert gas serves as a simple, effective protective packaging that can be easily implemented. Once opened, the target should be used promptly, making the packaging a single-use protective barrier that is both inexpensive and highly effective.
4Ease of operation
If oxidation film forms on lanthanum target surface, then handling in air becomes possible, but electric conductivity is reduced and sputtering defects occur
Solution Approach 1:
The target is handled within the sealed container filled with inert gas, allowing operators to work with the target without exposing it to air. This maintains the target's surface quality and electric conductivity while still enabling complete handling and processing operations.
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 method effectively produces high-purity lanthanum with reduced impurities, preventing oxidation and segregation, thus enhancing the stability and performance of sputtering targets and metal gate films, leading to improved semiconductor device quality.
Implementation Method 1
molten salt electrolysis using potassium chloride, lithium chloride, and lanthanum chloride
Implementation Method 2
electron beam melting
Data Source
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AI summary
The present invention addresses the problem of providing a technique capable of efficiently and stably providing a method for producing high-purity lanthanum, the method characterized in that: a crude lanthanum oxide starting material having a purity of 2N-5N, excluding gas components, is used; the material is subjected to molten salt electrolysis at a bath temperature of 450-700°C to produce lanthanum crystals; the lanthanum crystals are subsequently desalted: and electron beam melting is then performed to remove volatile substances. The present invention also addresses the problem of providing a technique capable of efficiently and stably providing high-purity lanthanum, high-purity lanthanum itself, a sputtering target formed from high-purity material lanthanum; and a thin film for metal gates that has high purity lanthanum as the main component.