Vacuum Fundamentals

High-Vacuum Technology Course

Week 2

Paul Nash

HE Subject Leader (Engineering)

Vacuum Fundamentals

• Outstanding enrolments• Recap on last week• Vapour Pressure & Mean Free Path

• Measuring Vacuum

Vacuum Technology

Vacuum Fundamentals

• To consider the vapour pressure of a variety of materials and the effect this has on vacuum and pump down

• To be able to describe a variety of vacuum measurement techniques

Learning Objectives

Vacuum Fundamentals

What is a Vacuum?

• Ideal Vacuum – A space totally devoid of all matter.– Does not exist, even in outer space!

• Actual Vacuum (Partial Vacuum) – A space containing gas at a pressure below

the surrounding atmosphere or atmospheric pressure

• <760T @ sea level and 00 C with no humidity

Vacuum Fundamentals

Common Vacuum Units There are many

varied units that are used to specify pressures

The Torr, the Bar and the Pascal are in common use...

.. but the Pascal is the SI recommended unit for pressure and so is the best choice for documentation

1 Atmospheric pressure is 760 mm Hg = 1 Bar = 105 Pa

1 Torr = 1 mm Hg 1 Torr = 1/760 of an

atmosphere = 132 Pa 1 milliTorr = 0.13Pa = 1

μmHg

1mbar = 1/1000 Atm = 0.76 Torr = 100Pa

1 Pa = 7.6 milliTorr = 7.6 μmHg

Vacuum Fundamentals

Low or Rough Vacuum

760 Torr to 1Torr

Medium Vacuum 1 Torr to 10-3 Torr

High Vacuum 10-3 to 10-7 Torr

Ultra-high Vacuum (UHV)

Below 10-7 Torr

Ranges of Vacuum

Vacuum Fundamentals

Vacuum Fundamentals

Pressure and Molecular Density• Molecules of gases tend to spread out, evenly applying force to the

containment chamber• A larger volume, with the same number of molecules present, would

be at lower pressure than a smaller one• Boyle’s Law - a relationship between pressure and volume

Vacuum Fundamentals

Kelvin Scale

C = .555 * (F – 32)

F = 1.8 * (C + 32)

K = (C + 273)

Vacuum Fundamentals

Charles’ LawVolume and Temperature

Vacuum Fundamentals

Combined Gas Law

• The relationships between pressure, temperature, and volume given in Boyle’s, Charles’, and Gay-Lussac’s Law for a constant number of gas molecules can be taken together as the Combined Gas Law.

• This law can be used two of the 3 properties are known to find the third.

(P1 * V1) / T1 = (P2 * V2) / T2

Vacuum Fundamentals

Vapour Pressure

Evaporation is the process where a liquid changes to a gaseous phase

In an open environment, liquids continuously evaporate

In a closed environment, eventually an equilibrium condition occurs where evaporation and condensation rates become the same. This occurs when the air becomes saturated.

Vacuum Fundamentals

Vapour Pressure

The vapour pressure of a substance in a chamber is important for a number of reasons.

• Possibility of vapourisation of the substance under low pressure– May add to gas load of system

• Use of vaporisation for processing– Physical evaporative coatings

Vacuum Fundamentals

Typical Vapour Pressures

Vacuum Fundamentals

Typical Vapour Pressures

Vacuum Fundamentals

Molecular Density and Mean Free Path

• Gas molecules collide with one another• Lower pressure results in fewer molecules per unit volume.

Vacuum Fundamentals

• The average distance travelled by a molecule between collisions is termed as the Mean Free Path:– For air at room temperature…

Molecular Density and Mean Free Path

PpathfreeMean

3104.6

Where P = Pressure in mBar

This means that the Mean Free Path is about 6x10-6 cm at atmosphere and 64 metres at 10-6 mBar

Vacuum Fundamentals

Vacuum Fundamentals

Where Do We Use Vacuum in

Manufacturing?

Vacuum Fundamentals

Common Uses of Vacuum

• Light Bulbs– A vacuum pump removes oxygen from a light bulb so

that the filament won’t “burn out” (oxidation)

• Food Processing– Vacuum sealing eliminates oxygen from food

containers to preserve the contents

• Plastics Manufacturing– Vacuum-forming “draws” plastic sheets into shapes

such as “blister packs”

Vacuum Fundamentals

To Retain a Clean Surface

• Objective – Clean surfaces

• Applications: – Friction – Adhesion– Emission studies – Materials testing for space

Vacuum Fundamentals

To Create Desired Features

• Objective – Create Insulators

• SiO2

• SiN2

– Create Conductive Layers

• Evaporative Coatings• Sputtered Coatings

– To etch or remove material

• Plasma Etch• Reactive Ion Etching

Sputtering Coating Systemhttp://www.teercoatings.co.uk

Vacuum Fundamentals

To Visualize Nano-features• Objective

– View extremely small Objects

• Scanning Electron Microscopy

• Electron beam strikes object being viewed

• Backscatter of electrons is used to “image”

– Atmospheric molecules present may be “hit” by the beam

http://en.wikipedia.org/wiki/Image:SEM_chamber1.JPG#file

Vacuum Fundamentals

Measuring Vacuum

Vacuum Fundamentals

Vacuum GaugesThere are 3 phenomena used to measure

vacuum:

MechanicalDisplacement of materials

TransportMovement of gases

IonisationIon currents

Vacuum Fundamentals

Vacuum Gauges• Vacuum systems must

be monitored constantly to ensure satisfactory performance, but manufacturers seem to be reluctant to provide gauges which allow this to be done

• Many different types of gauges are available because each only covers a limited range of pressures

• Never trust a gauge unless you can check it independently

Range of gauge utility

Vacuum Fundamentals

Pirani gauge• The Pirani is a

dedicated low vacuum gauge device

• The resistance of the hot wire changes with the rate of heat loss (conduction) to the gas

• The Wheatstone bridge then measures the change in resistance of the hot wire

• Pirani’s are rugged and generally reliable and rarely need attention

Schematic Circuit for a Pirani (hot wire) gauge

Vacuum Fundamentals

Pirani calibration

• The calibration of a Pirani depends on thermal conductivity and so on the actual gas in the system

• Beware when using a crystal spectrometer as gases leaking from the counter tubes will degrade the accuracy of the Pirani gauge

Correction Curve for Pirani Gauges

Vacuum Fundamentals

Penning (Cold cathode) Gauge

• A Penning gauge measures the ion current flowing from the cathode to the anode

• The magnetic field increases sensitivity by making the ions spiral as they travel to cause secondary ionization

• Beware - a Penning gauge reads zero current when the pressure is both very low and very high. The gauge must ‘strike’ to be operational

• Check with a Pirani gauge if in doubt

Penning gauges require routine cleaning and

testing

Vacuum Fundamentals

Capacitance Manometer

Gauge head on chamber Controller and digital read-out

Vacuum Fundamentals

Capacitance Manometer

• A = Annular electrode• D = Disk electrode• S = Substrate• G = Getter (in vacuum

space)• Differential capacitance

between annulus and disk depends on pressure difference between Test Chamber and “Getter”.

Vacuum Fundamentals

Ion gauges• Pressures lower than 10-5 Torr can

be measured with ion gauges

• Mass spectrometer gauges (residual gas analyzers) are a desirable extra. These can measure partial pressures of e.g helium (for leak testing) or of water vapor.