Welcome back. I'm Nan Jokerst. In this video we are going to talk about how to achieve low pressure also called high vacuum using a cryopump. Mechanical pumps alone cannot typically reach pressures low enough for thin film deposition of high quality, so we need to use another pump, a high vacuum pump in addition to the mechanical pump for high purity deposited thin films since air molecules in the vacuum chamber will introduce impurities in the film. However, the high vacuum pumps that will evacuate the chamber to a much lower pressure don't function at atmospheric pressure. So we need to start with the mechanical pump and then use a high vacuum pump after the mechanical pump. The two high vacuum pumps that we will present in this course are cryogenic pumps and turbo molecular pumps. The cryogenic pump or for short the cryopump is capable of reaching much lower pressures than a mechanical pump alone. Here's what we do. First, we rough pump the system with a mechanical pump and then after that we use the cryopump. The cryopump relies on a phenomenon that we see in our everyday lives, condensation, as shown on this glass of water. As a surface is cooled, gas molecules in the air are also cooled and some change state from gas to liquid. To condense all of the molecules in the air means that we need to use a very cold surface because many of these molecules condense at very low temperatures. A cryopump uses a compressor similar to a refrigerator with one big difference, a cryopump uses helium as a coolant. A cryopump is typically cool to 15 degrees Kelvin which is minus 258 degrees C. A cryopump can reach pressures down to 10 to the minus eighth torr or equivalently 1.3 times 10 to the minus 6 Pascals or even lower. The cooled region of the cryopump has large surface areas because more surface area means more trapped air molecules. Arrays of helium cooled metal fins called baffles provide that frozen surface area as shown here. Some areas in the pump are even covered with activated carbon which is porous and has extremely large surface area to trap even more air molecules. Here's a good question, where do the air molecules go that are frozen on the cryopump? A very good question because the cryopump has no outlet. Well the air molecules stay frozen and trapped in that pump until at some point all of the surface area on the metal fins and all of the pores in the carbon become filled with trapped air molecules. The term for this, is that the pump is saturated. When the pump is saturated it will no longer pump efficiently. This reduction in efficiency is obvious to the user because it either takes longer for the vacuum chamber to pump down to low pressure or the vacuum chamber will not pump to a lower pressure at all. Sometimes also the user will observe on the cryopump thermometer that the internal temperature of the pump begins to rise. When the pump is saturated, it needs to be regenerated. To regenerate the pump, it's filled with ultra high purity nitrogen gas which is slightly heated and the pump is allowed to slowly warm up to room temperature. This heating means that the metal fins can't freeze those air molecules any longer and the trapped air molecules evaporate and are then pumped out of the system with the mechanical pump. Think of it as hanging your clothes out to dry, the water comes off the clothes. Once this process is complete, the cryopump is as good as new and ready to pump efficiently again. One characteristic of cryopumps is the crossover pressure or the pressure which the user changes from a mechanical rough pump to the cryopump. As an example, if a cryopump has a 150 torr liter cross-over rating then we take that cross-over rating and divide by the chamber volume to calculate the cross-over pressure. That's the maximum pressure for cross-over from the mechanical pump to the cryopump or you'll be freezing too many air molecules and you'll have to regenerate too quickly. Cryopumps are one high vacuum pump option. Be sure also to take a look at the turbo pump video to learn about another commonly used high vacuum pump. I hope you enjoyed this lesson. Thank you for joining me today.