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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.