Cryopump: Basic Function

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From the course by Duke University

Nanotechnology: A Maker’s Course

224 оценки

Duke University

Nanotechnology: A Maker’s Course

224 оценки

How can we create nano-structures that are 10,000 times smaller than the diameter of a human hair? How can we “see” at the nano-scale? Through instruction and lab demonstrations, in this course you will obtain a rich understanding of the capabilities of nanotechnology tools, and how to use this equipment for nano-scale fabrication and characterization. The nanoscale is the next frontier of the Maker culture, where designs become reality. To become a Nanotechnology Maker pioneer, we will introduce you to the practical knowledge, skills, and tools that can turn your nanotechnology ideas into physical form and that enable you to image objects at the nano-scale. This course has been developed by faculty and staff experts in nano-fabrication, electron beam microscopy, and nano-characterization through the Research Triangle Nanotechnology Network (RTNN). The RTNN offers training and use of the tools demonstrated in this course to schools and industry through the United States National Nanotechnology Coordinated Infrastructure program. The tools demonstrated in this course are available to the public through the RTNN.

From the lesson

Nanofabrication: Vacuum Pumps and Thin Film Vacuum Deposition

In this module, we will discover how and why a vacuum environment is required in nanofabrication, compare the operation of three types of vacuum pumps, and look at vacuum deposition of thin films using three different methods: sputter evaporation, e-beam evaporation and thermal evaporation.

Mechanical Pumps: Basic Function7:19

Turbopump: Basic Function5:16

Cryopump: Basic Function5:45

Meet the Instructors

Nan M. Jokerst
J. A. Jones Professor of Electrical and Computer Engineering
Electrical and Computer Engineering, Duke University
Carrie Donley
Director of CHANL (Chapel Hill Analytical and Nanofabrication Laboratory)
Applied Physical Sciences, UNC Chapel Hill
James Cahoon
Assistant Professor
Chemistry, UNC Chapel Hill
Jacob Jones
Professor
Materials Science and Engineering, North Carolina State University

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.

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