Vacuum techniques

Dr. G. Mirjalili, Physics Dept. Yazd University Vacuum techniques Introduction Dr. G. Mirjalili Physics Dept. Yazd University Dr. G. Mirjalili, Physics Dept. Yazd University Introduction to Vacuum • What is vacuum ? – The official definition is: “Low Pressure” – Since “Low” is a relative term, “Low Pressure” is relative to atmospheric pressure. • So . . . what is pressure? – Pressure is “Force per unit area”. • pounds/square inch • newtons/square meter • (example: hand in sand) Dr. G. Mirjalili, Physics Dept. Yazd University Introduction to Vacuum • The pressure exerted by a gas in a closed, rigid container is caused by the gas molecules striking the surface of the chamber. – Gas: form of matter where the molecules can freely move in any direction, and it expands to fill its container • It is a combination of two factors: 1. how many gas molecules are in the chamber (molecular density), and 2. the kinetic energy (heat) of the gas molecules Dr. G. Mirjalili, Physics Dept. Yazd University Introduction to Vacuum • Therefore a vacuum (low pressure) can be created by: – reducing the number of gas molecules in the chamber (pumping), or – reducing the kinetic energy (temperature) of the gas molecules Dr. G. Mirjalili, Physics Dept. Yazd University Introduction to Vacuum • Atmospheric Pressure: – The earth is surrounded by an atmosphere consisting mainly of nitrogen and oxygen. – The force exerted by the atmosphere is 14.7 pounds per square inch at the surface of the earth (at sea level, 45O latitude, 0OC). – The 14.7 pounds (force) is the effect of the weight of the molecules in the air column above a one inch square of the earth’s surface extending to edge of the earth’s atmosphere. – Why is the atmospheric pressure lower as you go higher in elevation? Dr. G. Mirjalili, Physics Dept. Yazd University Introduction to Vacuum • Atmospheric Pressure: – altitude and atmospheric pressure are related with the following formula: – where P is in pounds per square inch (PSIA) – where A is the altitude above sea level in meters – The atmospheric pressure in a place (elevation 866 meters above sea level) is about 13.1 psia. – On top of Mt. Everest (8850m), the pressure is 4.3 psia, less than 1/3 of sea level!. 17000107.14 AP −×= Atmospheric Pressure vs. Altitude 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 0 2000 4000 6000 8000 10000 Altitude in meters above sea level PS IA Dr. G. Mirjalili, Physics Dept. Yazd University Dr. G. Mirjalili, Physics Dept. Yazd University Introduction to Vacuum • Net Pressure: – Molecules of air continuously striking the walls of a container exert a force on the walls. – If the container is open, then: • the force inside = the force outside • there is no “net pressure” on the chamber walls. – If the chamber is closed, and some of the gas inside is removed (pumped out), there will a be a net force pushing inwards on the walls of the container. • If the net force is greater than the mechanical strength of the container, there will be an implosion. Dr. G. Mirjalili, Physics Dept. Yazd University Introduction to Vacuum • Net Pressure: – Collapsed rail car due to condensing steam. • Dr. G. Mirjalili, Physics Dept. Yazd University Introduction to Vacuum • What is an ideal (perfect) vacuum ? – A space containing nothing – Pressure = 0 psia – Does this exist ? • Practically, no • Deep space is a close approximation ( < 10-20 psia) – What are some reasons why this does not exist in the practical world? • An actual vacuum is any pressure below atmospheric pressure. Dr. G. Mirjalili, Physics Dept. Yazd University Short History of Vacuum • Evangelista Torricelli - 1643 – developed a suction pump to remove water from a well – developed the first crude barometer – pressure unit (torr) is named after him Dr. G. Mirjalili, Physics Dept. Yazd University Short History of Vacuum • Short history: Otto von Guericke - 1663 – invented a piston vacuum pump – Magdeburg hemispheres demonstration: • two bronze hemispheres were put together, the air was pumped out, and two teams of horses could not pull them apart • impressed the socks off the emperor! Dr. G. Mirjalili, Physics Dept. Yazd University Short History of Vacuum • Edison - 1879 – used vacuum in his first electric lamp • early 1900’s – continued development of vacuum pumps and gauges to support new vacuum tube technology • 1950’s – development of diffusion pump, ion pump, and ion gauge opened the door to high and ultra-high vacuum: < 10-6 torr • 1970’s – turbomolecular pump and cryo pumps invented Dr. G. Mirjalili, Physics Dept. Yazd University Why is a Vacuum Needed? To move a particle in a (straight) line over a large distance 1: Dr. G. Mirjalili, Physics Dept. Yazd University Why is a Vacuum Needed? Contamination (usually water) Clean surface Atmosphere (High)Vacuum To provide a clean surface2: Dr. G. Mirjalili, Physics Dept. Yazd University Why Use Vacuum? • Cleanliness: low pressure => low number of molecules of potential contaminants – extend formation time for native oxides – reduce or eliminate impurities incorporated during processing • Plasma generation: plasmas can easily be created and sustained in a low pressure environment – used for etching, deposition, and ion implantation • Lower molecular interference: – increase mean free path for ions used in sputtering Dr. G. Mirjalili, Physics Dept. Yazd University Why Use Vacuum? • Low friction: – reduce heat dissipation requirements for processes. • Thermal insulation: – i.e. thermos bottle • Promote evaporation: – materials can be evaporated at lower temperatures by reducing the pressure • Mechanical advantage: – use pressure differences to hold items in place or to transport them from one place to another Dr. G. Mirjalili, Physics Dept. Yazd University Uses for Vacuum: Non- Semiconductor • freeze drying of foods and pharmaceutical products • optical lens coatings • vacuum storage to retard spoilage or oxidation (rust) • vacuum tubes and cathode ray tubes • vacuum cleaners • mechanical advantage: – suction cups – transport (bank drive-up window) Dr. G. Mirjalili, Physics Dept. Yazd University Uses for Vacuum: Semiconductor • Layering (CVD, sputter, evaporation) – reduce impurities – allow plasma generation – increase mean free path • Dry etching – plasma etching or Reactive Ion Etching • Ion implantation (doping) • Vacuum clamping (chucks, wands, robot arms) used to manipulate wafers by touching backside only. – Used in virtually all process tools Dr. G. Mirjalili, Physics Dept. Yazd University Pressure Measurement Dr. G. Mirjalili, Physics Dept. Yazd University BAROMETER WATER MERCURY 760 mm Mercury: 13.58 times heavier than water: Column is 13.58 x shorter : 10321 mm/13.58=760 mm (= 760 Torr) 10.321 mm 29,9 in Dr. G. Mirjalili, Physics Dept. Yazd University Pressure Measurement • Barometer: – Torricelli placed a glass tube that was sealed at one end in a beaker of mercury (Hg). – The level of Hg in the tube went up and down as the surrounding pressure was changed. – At sea level, the atmospheric pressure will support a column of Hg 760mm high. – 1 torr = 1 mm Hg = 1/760 atmospheric pressure Dr. G. Mirjalili, Physics Dept. Yazd University Pressure of 1 standard atmosphere AT SEA LEVEL, 0O C AND 45O LATITUDE CGS →P=dyne/cm2=bar P=ρgh=13.6×98×76=1012928bar≈10 6bar ≈ 1013 mbar MKS →P=N/m2 =pascal P=13600 ×9.8 ×.76=101292.8 ≈105 pascal 100 pascal=1 mbar=.76 Torr Dr. G. Mirjalili, Physics Dept. Yazd University Pressure Equivalents Atmospheric Pressure (Standard) = 0 14.7 29.9 760 760 760,000 101,325 1.013 1013 gauge pressure (psig) pounds per square inch (psia) inches of mercury millimeter of mercury torr millitorr or microns pascal bar millibar Dr. G. Mirjalili, Physics Dept. Yazd University THE ATMOSPHERE IS A MIXTURE OF GASES PARTIAL PRESSURES OF GASES CORRESPOND TO THEIR RELATIVE VOLUMES GAS SYMBOL PERCENT BY VOLUME PARTIAL PRESSURE TORR PASCAL Nitrogen Oxygen Argon Carbon Dioxide Neon Helium Krypton Hydrogen Xenon Water N2 O2 A CO2 Ne He Kr H2 X H2O 78 21 0.93 0.03 0.0018 0.0005 0.0001 0.00005 0.0000087 Variable 593 158 7.1 0.25 1.4 x 10-2 4.0 x 10-3 8.7 x 10-4 4.0 x 10-4 6.6 x 10-5 5 to 50 79,000 21,000 940 33 1.8 5.3 x 10-1 1.1 x 10-1 5.1 x 10-2 8.7 x 10-3 665 to 6650 Dr. G. Mirjalili, Physics Dept. Yazd University Vapor pressure of water at various temperatures T (O C) 100 25 0 -40 -78.5 -196 P (mbar) 1013 32 6.4 0.13 6.6 x 10 -4 10 -24 (BOILING) (FREEZING) (DRY ICE) (LIQUID NITROGEN) Dr. G. Mirjalili, Physics Dept. Yazd University Pressure Measurement • Other units of pressure: – inches of water, pascals, bar, atmospheres, and pounds per square inch. – Pascals and Bar are used in Europe – In the U.S. (and in this class) we mostly use torr written using scientific notation. • i.e. 5 x 10-6 torr, etc. • Important conversion, atmospheric pressure: – 1 atm. = 760 torr = 760 mm Hg = 406.8 in. H2O = 14.7 pounds/in2 = 1.013 bar = 101,325 Pa • Note: Potential confusing terminology! – High vacuum = low pressure – Low vacuum = high pressure (relatively) Dr. G. Mirjalili, Physics Dept. Yazd University Pressure Measurement • Vacuum Ranges: (depends on source) • Does the risk of implosion increase significantly as you move from low vacuum to ultra-high vacuum? • Which vacuum range(s) would a barometer be useful for? 10-14 10-12 10-10 10-8 10-6 10-4 10-2 100 102 103 Pressure (Torr) 103 105 107 109 1011 1013 1015 1017 1019 Density in molecules per cubic cm (at 25 OC) 109 107 105 103 101 10-1 10-3 10-5 10-7 Mean Free Path in meters (at 25 OC) High Vacuum Medium Vacuum Rough Vacuum Ultra-High Vacuum Atm Dr. G. Mirjalili, Physics Dept. Yazd University Vacuum Ranges • Low (rough) Vacuum: – atmosphere down to 1 torr, – clamping, non-industrial applications • Medium Vacuum: – 1 torr to 1 x 10-3 torr – CVD, plasma etch • High Vacuum: – 1 x 10-3 to 1 x 10-8 torr – ion implanters, sputterers, evaporators • Ultra-high Vacuum: – less than 1 x 10-8 torr – molecular beam epitaxy, research instruments Dr. G. Mirjalili, Physics Dept. Yazd University Additional Vacuum Terminology • Molecular Density: the number of particles per unit volume of a gas (particles/cm3) – what is molecular density as you approach an ideal vacuum? • Mean Free Path: the average distance a molecule of a gas can travel in a straight line before striking another molecule or the chamber wall – what is the mean free path as you approach an ideal vacuum? Dr. G. Mirjalili, Physics Dept. Yazd University Vacuum Terminology • Base (Ultimate) Pressure: the lowest pressure a vacuum system can achieve – no gas is flowing into the system – used at the beginning of a process to remove impurities – what might limit the base pressure of a system? • Process (Operating) Pressure: pressure in a system with gasses flowing into it at equilibrium – is process pressure higher or lower than base pressure?