The pressure inside a vacuum chamber can vary significantly depending on the specific application and the type of vacuum being used. The pressure ranges from atmospheric pressure down to ultra-high vacuum levels, which can be as low as 10-11 mbar or 7.5-11 Torr. The choice of pressure depends on the process requirements, such as sample coverage, thin film quality, and the need for inert gas environments. Accurate pressure measurement is crucial and is typically achieved using high-quality pressure gauges.
Key Points Explained:
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Pressure Ranges in Vacuum Chambers:
- Rough/Low Vacuum: 1000 to 1 mbar (760 to 0.75 Torr)
- Fine/Medium Vacuum: 1 to 10-3 mbar (0.75 to 7.5-3 Torr)
- High Vacuum: 10-3 to 10-7 mbar (7.5-3 to 7.5-7 Torr)
- Ultra-High Vacuum: 10-7 to 10-11 mbar (7.5-7 to 7.5-11 Torr)
- Extreme High Vacuum: < 10-11 mbar (< 7.5-11 Torr)
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Importance of Pressure in Deposition Processes:
- During thermal evaporation, the pressure inside the chamber plays a critical role in determining the quality of the thin film deposited. The pressure must be low enough to ensure that the mean free path of particles is longer than the distance between the source and the substrate, typically around 3.0 x 10-4 Torr or lower.
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Pressure Gauges and Measurement:
- For accurate pressure measurement, high-quality pressure gauges are essential. In the DTT model deposition system, a full-range pressure gauge made by Leybold Company is used, capable of displaying pressure from atmospheric levels down to 10-9 Torr.
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Applications and Suitable Vacuum Levels:
- Medium Vacuum (< 1, > 10-3 Torr): Suitable for some welding and machining applications.
- High Vacuum (< 10-3 Torr, > 10-8 Torr): Suitable for high-vacuum furnaces.
- Ultra High Vacuum (< 10-8 torr): Used for processes requiring extremely clean environments, such as drying out surface contaminants and cleaning liners.
- High Pressure (> 760 Torr): Suitable for furnaces using gases or internal atmospheres at positive or higher than atmospheric pressure.
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Effect of Vacuum on Process Environment:
- For a clean environment in a process chamber, an ultra-high vacuum can be more effective than using inert gas. An evacuated process chamber can routinely reach a residual pressure below 0.0001 mbar if given sufficient pumping time, whereas inert gas flushing may result in higher impurity partial pressures, potentially up to 0.1 mbar due to various practical constraints.
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Maximum Possible Vacuum:
- The maximum possible vacuum inside a chamber can be equal to 760 mmHg (atmospheric pressure), but practical applications often require pressures much lower than this, depending on the specific process and equipment capabilities.
In summary, the pressure inside a vacuum chamber is a critical parameter that influences the outcome of various processes, from thin film deposition to welding and machining. The choice of pressure range depends on the specific requirements of the process, and accurate measurement and control of pressure are essential for achieving the desired results.
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