vacuum techniques

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Information about vacuum techniques
Education

Published on January 4, 2009

Author: msmurali

Source: authorstream.com

Vacuum Techniques : Vacuum Techniques Introduction : Introduction The term vacuum refers to the condition of an enclosed space that is devoid of all gases or other material content. It is not experimentally feasible to achieve a “perfect” vacuum, although one can approach this condition extremely closely. Common Pressure Units : Common Pressure Units 1 Torr = 1 mm Hg at 0°C, standard gravity 1 atm = 760 Torr 1 micron = 1 mm Hg = 10-3 Torr = 1m Torr Mean free path : Mean free path The mean free path ? is the average distance traveled by a gas molecule between collisions with other molecules. Where d is the molecular diameter “T” temp in Kelvin, “P” pres. in Torr,”k” Boltzman’s const. Gas FlowImportance of mean free path : Gas FlowImportance of mean free path Viscous flow mfp << size of system (D) gas - gas collisions dominate molecules “drag” one another Intermediate flow (transition) mfp comparable to size of system (D) complicated flow Molecular flow mfp >> size of system gas - wall collisions dominate molecules move independently of one another Vacuum measurements : Vacuum measurements Vacuum System Components : Vacuum System Components VACUUM PUMPS Rotary mech. pump : VACUUM PUMPS Rotary mech. pump 1.Begin expansion cycle. 2. Seal off expanded volume 3.Compresses gas out exhaust Safety considerations for a Rotary oil pump : Safety considerations for a Rotary oil pump Trap vapors entering pump-(to prevent degradation of pump oil and seals Do not pump on atmosphere-(the pump would over heat) Diffusion pumps : Diffusion pumps Principle of Operation -momentum transfer by vapor jet stream Individual molecules are “pushed” toward exhaust by jet stream. Hence the pump works in the molecular flow regime Applicable range 10-2 to 10-6 Torr Safety consideration for a Diffusion pump : Safety consideration for a Diffusion pump Rotary oil pump must be on to reduce the pressure to < 10-2 Torr(Otherwise hot oil would combust in presence of Oxygen) Diffusion pump must be cooled with water or air(to prevent overheating) Vacuum line must be provided with a trap to prevent migration of diffusion pump oil Other Pumps : Other Pumps Turbomolecular pump: Ultra-fast fan blades knock molecules out of vacuum system Cryo pump: molecules are frozen out Sorption pump: molecules diffuse into absorbing material Sputter ion pump: molecules are ionized and buried Valves : Valves High vacuum Valve The d.p. is connected to the coating chamber via a combined oil baffle and isolation valve which permits the d.p. to be kept under vacuum at operating temp. when the coating chamber is at atm. Pressure. Oil Vapor Baffle Valve The d.p. is adequately baffled against the direct entry of oil vapor molecules into vacuum chamber Two Way Valve Air Admittance Valve Cold Trap : Cold Trap Purpose Prevents pump oil from contaminating vacuum system prevents organic solvents from reaching pump acts as a pump by condensing molecules Principle of operation freeze out contaminants Pressure gauges : Pressure gauges Barometer Uses the height of a column of mercury to measure ambient atmospheric pressure Manometer Uses the height of a mercury column to measure system pressure relative to ambient atmospheric pressure Pressure gauges contd...Pirani gauge : Pressure gauges contd...Pirani gauge Principle pressure dependence of thermal conductivity At higher pressures , the temperature of the sensing wire decreases as the thermal conductivity of the gas increases. Consequently the resistance of the wire decreases and the current thru the unbalanced bridge indicates the change in pressure. Pressure gauges contd...Ionization gauge : Pressure gauges contd...Ionization gauge Principle of operation ionization of residual gas by a high voltage which results in a current of charged particles Vacuum pump oils/grease : Vacuum pump oils/grease Rotary pump oil molecular distilled oil Properties: - lubricants - doesn’t mix with water - highly resistant to corosive gases Help : - better yield - better pump life - good recovery Oils and grease contd. : Oils and grease contd. Diffusion pump oils sillicon fluids/poly phenyl-ether fluids (e.g. DC 702,704,705) properties: - these silicon fluids are not oxidised by air at working temperatures - not hydrolised by water vapor Vacuum oils/grease contd. : Vacuum oils/grease contd. Vacuum grease silicon grease use: - to lubricate seals and act as sealant properties: - polymer based lubricant - paste material which doesn’t flow - consistency from -40 to 260 deg. C - used upto 10-8 torr - insoluble in water,methanol, ethanol, acetone etc. Slide 21: Boiler Foreline Valve R.P.Vent Valve Roughing Valve Air Vent Valve Water Cooled Diffusion Pump H.V./Baffle Valve Base Plate Penning Gauge Pirani Gauge 2 A Typical Diffusion Pumped Deposition System Slide 22: Boiler Water Cooled Diffusion Pump Vacuum Pumps Rotary Mechanical Pump Slide 23: Boiler Foreline Valve R.P.Vent Valve Roughing Valve Air Vent Valve H.V./Baffle Valve Base Plate Valves Slide 24: Boiler Penning Gauge Pirani Gauge 2 Vacuum Gauges Pirani Gauge 1 Slide 25: WATER SUPPLY ON R.P. ON OPEN CLOSED CLOSED CLOSED Getting started Slide 26: WATER SUPPLY ON R.P. ON OPEN CLOSED CLOSED CLOSED BOILER ON Never let a hot D.P. See a high or atm. pressure D.P. ON/Fore-line pumping Slide 27: R.P. ON BOILER ON D.P. ON CLOSED CLOSED OPEN CLOSED CLOSED BELL JAR CLOSED WATER SUPPLY ON Rough Pumping the chamber Slide 28: WATER SUPPLY ON R.P. ON OPEN CLOSED CLOSED OPEN BOILER ON Pumping to High Vacuum References : References K.L.Chopra, Thin film phenomena L.Holland, vacuum deposition of thin films L.Maissel and R.Glang, Handbook of thin film technology

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