Outgassing in a vacuum chamber is a concern for several reasons:
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Contamination: Outgassing refers to the release of gases, vapors, or volatile substances from materials when they are exposed to a vacuum. These released substances can contaminate the vacuum environment and affect the performance of sensitive equipment or experiments. In applications such as semiconductor manufacturing, optics, or space simulations, even minute levels of contaminants can have a significant impact on the desired outcomes.
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Pressure Changes: Outgassed substances can increase the pressure within the vacuum chamber, leading to an undesired increase in overall pressure. This increase can affect the accuracy of pressure-sensitive measurements or disrupt the functioning of delicate instruments that rely on a specific pressure range.
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Film Formation: Some outgassed substances can condense on surfaces within the vacuum chamber, leading to the formation of thin films. These films can interfere with the functioning of equipment, optics, or sensors, affecting their performance or accuracy. They can also alter the characteristics of surfaces or materials being processed or analyzed within the chamber.
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Degradation of Vacuum: Outgassing can introduce additional gases into the vacuum chamber, reducing the purity of the vacuum. This can compromise the efficiency of processes that rely on a high-quality vacuum, such as thin film deposition or particle accelerators. The presence of outgassed gases can also interact with other materials or substances in the chamber, leading to chemical reactions or undesired side effects.
To mitigate these concerns, various measures are taken to minimize outgassing in vacuum chambers. These measures include using materials with low outgassing rates, employing special coatings or surface treatments, implementing proper cleaning and preparation procedures, and using dedicated pumps or traps to remove and collect the outgassed substances.
Anything that goes inside the chamber must be outgas compliant. Low outgassing is defined as collected volatile condensable materials (CVCM) <0.1% and total mass loss (TML) < 1%. Note, higher TML is acceptable if a large portion of the TML is from water vapor (WV) as follows:
• If CVC <0.1% and TM < 1%, the material passes
• If CVC <0.1% and TM > 1 %, the material can pass if the TML-WV <1%
• If CVC > 0.1% or TML-WV > 1%, the material fails.
Below are some materials guidelines:
• Generally, metals and ceramics are good.
• PTFE/Teflon, commonly used for wire housing, is good
• Heat shrink/strain relief deserves some investigation; outgas-compliant heat shrinks exist e.g. Sumitube B2
• Many adhesives are acceptable: Kapton tape, Nusil CV-1142 and Araldite 2014 A/B are regularly used
Certain materials are forbidden as they can damage the chamber or the unit under test.
• Zinc and its alloys
• Cadmium and its alloys
• Mercury and its alloys
Pure tin includes the risk of tin-whiskering, and this risk should be understood by the engineer choosing to use tin. Generally, this is avoided through the use of leaded solder.
Thermal interface material is used to increase thermal contact between the interface plate/unit and the platen. Many TIMs are acceptable, including but not limited to:
• Many types of Sarcon
• Graphite sheets, provided the bonding agent is outgas compliant
• A Kapton tape carrier is acceptable
• Note: graphite is prone to breaking apart and creating FOS
• Aluminum or indium foils
References:
https://outgassing.nasa.gov/
http://esmat.esa.int/services/outgassing_data/outgassing_data.html