Why Is There Vacuum?(The sequel to Bill Cosby’s “Why Is There Air?”)

Matthew C. DeLongUniversity of Utah

OptoElectronic Materials Laboratory7 January 2008

Ranges of Vacuum

• Low: 1 atm to 1 Torr• Medium: 10-3 ( 1 m) to 1

Torr• High: 10-8 to 10-3 Torr

• Ultra High: 10-12 to 10-8 Torr• Extreme: < 10-12 Torr• Note: low vacuum ↔ high

pressure

• Drying, drinking straws• Sputtering

• Thermal evaporation, e-gun, SEM

• STEM, FIM, AES, SIMS• Anti-particle

accumulators, space simulation

Pressure: Units of Measure

• Pressure exerted by a column of fluid:• P ≡ F/A = mg/A = ghA/A = gh h • 1 Atm (mean sea level) = 760 Torr = 1013

mBar = 1.01x105 Pa = 101.3 kPa = 14.7 psi = 34 ft. water

• Average atmospheric pressure in SLC is about 635 Torr, 12.3 psi, 28.4 ft water…

“Kinds of Pressure”

• Gauge Pressure: measured with respect to ambient.

• Absolute pressure: measured with respect to vacuum

• Car tires, basketballs, boilers, LN2 tanks, JFB compressed air supply…

• Vacuum systems, cathode ray tubes, light bulbs, barometers

Measurement Techniques

• Low

• Medium• High

• Ultra High• Extreme

• Mechanical (Bourdon), Hg column, capacitance

• Thermocouple, Pirani• Ionization [hot and cold (Penning)

cathode]• Ionization (hot cathode: Bayard-Alpert)• Modulator Bayard-Alpert

Bourdon Gauge (Mechanical)

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”.

Heat Transfer of Gases

• Conductivity is linear in pressure over about 2 orders of magnitude.

• Molecular flow regime

• Pirani and thermocouple gauges

Ionization gauges

• Hot cathode: more sensitive; less forgiving• Cold cathode: less sensitive; more forgiving

Chambers et al. P.84

Mean Free Path in Gases

With sufficient accuracy for approximate calculations we may take:

λ = 7 x 10-3/p mbar-cmλ = 5 x 10-3/p Torr-cmλ = 5/p μmHg-cm

Roughing pump comparisons: Oil Sealed Pumps

Type Advantages DisadvantagesRotary vane Low ultimate

pressure.Low costLong pump life.

Backstreams oil. Produces hazardous waste.

Rootes Lobe Very high pumping speed

Frequent maintenance. Requires a purge gas. Requires a backing pump. Must be absolutely horizontal.

Rotary piston High volumeLow cost

Noise. Vibration Safety Valve.

Roughing pump comparisons: Dry Roughing Pumps

Scroll Clean.Low "dry" ultimate pressure.Easily serviceableQuiet.Technology is well known.

Limited bearing life. Limited scroll life. Permeable to small gases. Not hermetically sealed. Clean applications only.

Diaphragm Low cost.Quiet.Easily serviced.

Low pumping speed. High ultimate pressure. Frequent service required.

Hook and Claw

No backstreaming.Low ultimate pressure

Expensive

Screw rotor Low ultimate vacuum.Less maintenance than hook & claw

Expensive

Dry piston Low ultimate pressure Expensive

Sorption Clean Requires LN2.

Rotary Vane Mechanical Pump

• Robust• Inexpensive• Operates to

ambient pressure

• Single stage and two stage

Sorption Pump

• Clean: no oil• Very inexpensive:

170,000 Torr-liters for $1000 + 8.5 l LN2

• Requires LN2• Air adsorbs onto

zeolite at 77K• 10-3 Torr capability

Oil Vapor Diffusion Pump

Vacuum system

• Robust (silicone oil!)

• Low maintenance: no moving parts

• Requires backing• 10-3 – 10-7 Torr

Turbomolecular Pump

• Requires backing: Operates only <1 Torr

• Clean: no oil• Expensive:

Approximately triple the cost of a rotary vane mechanical pump and oil diffusion pump

• Limited lifespan

Getter pump• Low maintenance:

no moving parts• 10-4 – 10-10 Torr• Requires backing• Clean: no oil• Based on

chemical reaction of “air” with very reactive metals

Vac-Ion Pump (Sputter/Getter)

• Clean: no oil• 10-4 – 10-10 Torr• Not cheap!• Require backing

References• A. Chalmers, B.K. Fitch, and B. S. Halliday, Basic

Vacuum Technology, IOP Publishing, Bristol (1998). TJ/940/C45/1998.

• D. Hucknall, Vacuum Technology and Applications, Butterworth-Heinemann, Oxford (1991). TJ/940/H83 (1991).

• Vacuum Equipment, Granville-Phillips Co., Boulder CO. TJ/940/G7.

• R. R. LaPelle, Practical Vacuum Systems, McGraw-Hill, New York (1972).

• David Joy, “New Lecture 3” on course website.