September 17, 2020
3 MIN READ
To deposit high quality thin films for optical markets, such as Anti-Reflective and Filter coatings, a high vacuum pressure, that is 10-3 mbar to 10-7 mbar, is essential. The same is true for metal deposition, including Aluminium and Copper, required by the Semiconductor sector. While there are a few different pumps that can achieve this pressure, we share why using cryo pumps will help you stay at the leading edge of your industry.
At first sight there are several pump types that can achieve this pressure, namely:
• Cryo pumps
• Diffusion pumps
• Turbomolecular pumps
As with any high vacuum pump, a suitable forevacuum pump will also be needed.
Diffusion pumps, whilst offering pumping speeds up to 50,000 l/s, create oil contamination and are generally not suitable for the high-quality films that are demanded by the optical and semiconductor sectors. One could add baffles, but these can reduce the actual pumping speed by up to 50 per cent. Moreover, diffusion pumps have relatively high running costs in terms of power and cooling water. In addition to the forevacuum pump, an additional smaller holding pump and bypass line are needed to maintain the diffusion pump when the system is initially evacuated from atmosphere.
Whilst turbomolecular pumps are oil free, the maximum pumping speed is around 3000 l/s, which is generally too low for high-throughput processes. Moreover, these pumps are poor at pumping water vapor and light gases, crucial for many coating applications.
The lighter gases such as hydrogen would not condense at 20K, and so a charcoal coating is applied to “cryo-absorb” these species. This again allows higher pumping speeds for these gases, relative to a turbomolecular pump. These gases will eventually saturate the 2nd stage, resulting in a rise in ultimate pressure.
The pump then needs to be regenerated by heating the stages whilst pumping away the released gaseous species with a small fore vacuum pump — an oil free pump such as a scroll is recommended. This prevents contamination of the pump cryo panels. Regeneration typically takes 2 hours and can be incorporated as part of the routine maintenance. A cryo pump can meet all these demands. As the name suggests, the pump employs a refrigeration principle. Helium-4 is compressed and expanded to achieve extremely low temperatures, resulting in the condensation of gaseous species. The cryo pump is classed as an entrapment pump. The schematic below illustrates a typical cryo pump system:
The next diagram shows how the pump condenses gases depending on their freezing point:
There are two distinct temperatures, which are achieved by compressing high purity Helium-4. The first stage traps water vapor, and offers an extremely good pumping speed, producing high quality films. The pump-down time of a high vacuum system is very dependent on the degassing of absorbed water molecules, so this high pumping phase will reduce this crucial time demand.
The time between regenerations is obviously influenced by gas load, but is typically 7-10 days, depending on the process pressure.
Related: Take a closer look at Leybold's range of cryo pumps in our blog post, Cryogenic Pumping with COOLVAC: A Short Introduction.
Cryo pumps have four key advantages for thin film applications compared to diffusion and turbo pumps, namely:
No risk of oil contamination
Overall pumping speeds up to 60,000 l/s
Low running costs
High pumping speed for water vapor and light gases.
Do you have more questions? The Leybold team is always ready to answer your questions about vacuum pumps and cryo technology. Click the button below and contact us to get answers to your burning questions or explore tailor-made solutions for your vacuum needs.
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