E SC 412 Nanotechnology: Materials, Infrastructure, and Safety Wook Jun Nam
Unit 2 Infrastructure Lecture 6 More on Vacuum Systems, Part III
Unit Outline Infrastructure * What is it? * Why do we need it in nanotechnology? Facilities Equipment Systems * Vacuum Based Systems * Non-vacuum Based Systems
Lecture 6 Outline Vacuum pumps Vacuum leaks
Vacuum Pumps Pumps are selected and used based on a number of criteria, including: Vacuum range required Gases to be pumped Pumping speed Overall throughput Corrosive gas tolerance Cost Maintenance requirements Downtime Q. Do you get all attributes with all pumps? Operational physics dictates the performance
Typical Ranges for Vacuum Pumps Vacuum Pumps Pressure (Torr) 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 To ATM. Mech. Diffusion Cryo Turbo Public Domain: Generated by CNEU Staff for free use
Vacuum Pumps The apparatus used to remove gas particles from a process chamber Thereby, reducing pressure Vacuum pumps used in nanofabrication can be broken into two categories: Mechanical (Rough) pumps High vacuum pumps
Mechanical Pumps Capable of reducing pressure to the low vacuum state of approximately 10-3 Torr Mechanical pumps are used to initially reduce pressure in a high vacuum system Oil sealed rotary vane mechanical pumps are most common Recall, the amount of water and residual gas at 10-3 Torr
Oil Sealed Rotary Vane Pump Gas enters the chamber (A) and is compressed by the rotor (3) and vane (5) in region B and expelled to the atmosphere by the discharge valve (8) An airtight seal is made by one or more spring loaded vanes and closely spaced sealing surfaces The vanes and the surfaces between the rotor and housing are sealed by low vapor pressure oil, which also serves to lubricate the pump
Mechanical Pumps Advantages Can pump from 760 Torr Rugged construction Low maintenance Low operating cost Low initial cost Long life Small to very large size Disadvantages Backstream oil Noise Speed decreases below 10-1 Torr Sensitive to particulates Low frequency vibration
Backstreaming Potentially, a major source of contamination The process by which pump oil evaporates out of the pump and flows into a process chamber As chamber pressure decreases, backstreaming becomes a bigger threat Dependant upon the vapor pressure of the pump oil This problem is driving the industry switch to dry pumps
Pump Oil If vapor pressure requirements are not severe, mechanical pumps use standard hydrocarbon based oil: Known as mineral oil However, several characteristics make mineral oil unsuitable for use in high vacuum pumps Vapor pressure is too high Not stable in high oxygen environments At higher temperatures, hydrocarbons can break down or crack
Synthetic Oil Developed to overcome the shortcomings of mineral oil Synthetic oils have low vapor pressure, high viscosity index, a high degree of oiliness, and chemical inertness
Fluorochemicals Characterized by their inertness to a wide range of chemical compounds: Suitable for high oxygen environments We use two commercially available varieties of completely fluorinated oil: Fomblin Krytox
Mechanical Pump System TC1 Chamber Vent Valve Roughing Valve Molecular Sieve Trap Isolation Valve Mechanical Pump TC = Thermocouple gauge TC2 Public Domain: Generated by CNEU Staff for free use
High Vacuum Pumps High vacuum pumps establish the final vacuum in a process chamber 10-3 Torr High vacuum pumps are made of materials that will not out-gas into the system: 304 stainless steel Titanium Kovar Nickel Borosilicate glass Ceramics Tungsten Gold Oxygen-free high conductivity copper (OFHC)
High Vacuum Pumps High vacuum pumps come in several varieties: Oil diffusion Cryogenic Turbomolecular Ti sublimation pump
Diffusion Pumps A vapor jet pump which transports gas by momentum transfer on collision with the vapor stream Pump oil is heated in the boiler until it vaporizes
Diffusion Pumps The nozzles direct the vapor downward toward the water-cooled outer wall where it condenses and returns to the boiler Any gas that diffuses into the vapor stream is directed downward and ejected into a region of higher pressure Modern pumps have several stages that compress the gas to a successively higher pressure as it is transported to the outlet
Diffusion Pumps
Diffusion Pumps Advantages Reliable No moving parts. Inexpensive Relatively quick/constant pumping speed. High throughput Low cost Disadvantages Prone to backstreaming (dirty). Not suitable for corrosive or O 2 environments. Requires cryotrap Requires backing pump.
Silicone Based Oil A series of siloxane polymers Repeating units of silicon-oxygen groups with silicon bonds to side groups (methyl, phenyl, alkyl, chloro, etc.) A class of silicone pump oil, trisiloxane, is widely used in diffusion pumps for many reasons: Stability, resistance to oxidation, low vapor pressure However, their lack of adhesion and inability to form a film on steel limits their usefulness in other pumps
Diffusion Pump System TC1 Chamber Vent Valve Ion Gauge Roughing Valve Molecular Sieve Trap Hi-Vac Valve LN 2 Trap Diffusion Pump Mechanical Pump TC2 Foreline Valve Public Domain: Generated by CNEU Staff for free use
Cryogenic Pumps The capture of molecules on a cooled surface by weak van der Waals or other forces Based on cryo-condensation, cryo-sorption, and cryotrapping In essence, cryogenic pumps use liquid helium or liquid nitrogen to cool and segregate gases A capture pump
Cryogenic Pumps Cryopumps are designed with a central finned stack (an expander) Gas from the chamber collects on the vanes, removing material from the system, reducing pressure The expander is cooled as a compressor releases helium into it from the bottom Cryo-pumps can efficiently pump water vapor, which many other pump designs cannot
Cryogenic Pumps
Cryogenic Pumps Advantages High speed Very high throughput Clean Low operating costs Does not require backing Disadvantages Requires regeneration Periodic maintenance required Requires ultrapure coolant Possibility of explosion (gas dependant)
Cryopump System TC1 Chamber Vent Valve Ion Gauge Roughing Valve Molecular Sieve Trap 1 Compressor Cryopump TC2 Hi-Vac Valve Mechanical Pump Pump Body Roughing Valve Trap Valve 2 Trap Valve 1 Molecular Sieve Trap 2 Public Domain: Generated by CNEU Staff for free use
Turbomolecular (Turbo) Pumps Turbo pumps are similar in design to a jet turbine engine A series of blades with openings are mounted and rotated at very high speeds on a central shaft (24,000 to 40,000 rpm) Turbo pumps work on the principle of momentum transfer Similar to diffusion pumps
Turbomolecular (Turbo) Pumps Gas molecules from the chamber encounter the first blade and gain momentum from the collision with the rotating blades The momentum direction is downward to the next blade, where the same thing happens, resulting in gas removal from the chamber
Turbomolecular (Turbo) Pumps Public Domain: Generated by CNEU Staff for free use
Turbomolecular (Turbo) Pumps Advantages No Oil = No backstreaming Able to handle corrosive/high O 2 gases / - C series High throughput Fast start-up Constant speed Disadvantages Cost Short lifespan (1 year) Bearings Cannot operate at atmospheric pressure (needs a backing pump) Low speed for hydrogen and helium
Turbomolecular Pump System Chamber TC1 Vent Valve Ion Gauge Roughing Valve Molecular Sieve Trap 1 Hi-Vac Valve Turbo Pump Trap Valve 1 Mechanical Pump Foreline Valve TC2 Trap Valve 2 Molecular Sieve Trap 2 Public Domain: Generated by CNEU Staff for free use
Outline Vacuum Pumps Vacuum Leaks
Leaks in a Vacuum System Defined as a rise in pressure towards atmosphere after a vacuum has been achieved It is impossible to achieve a perfect vacuum: There will always be some type of leak Leaks are divided into two categories: Virtual Real
Real Leaks Any pathway that a gas can travel from the outside of a vacuum system to the inside These can include: A broken gasket An improperly installed gasket Malfunctioning pump
Virtual Leaks A completely internalized phenomenon The leak path originates on the inside of the system rather than atmosphere Causes Outgasing of materials Contamination Residual water vapor Backstreaming
Water Contamination Concerns Water deserves special attention: It exists as a vapor in air at relatively large amounts Water vapor tends to stick to the sidewalls of process chambers and is particularly difficult to remove in a vacuum system It is common to see the pump rate slow at various vapor pressure plateaus. The water plateau is 10-4 Torr In general, the lower the vapor pressure, the longer a liquid will take to evaporate
Vapor Pressure of a Liquid The vapor pressure of a liquid is caused by the fact that all liquids have a tendency to evaporate and all gases have a tendency to condense back to their liquid form. These trends come into equilibrium in a closed system, which is at some temperature T, thereby establishing the liquid s vapor pressure at the temperature T. A high vapor pressure means the liquid s molecules like to escape from the liquid. Either evaporation or condensation will win if the system is not closed. Evaporation will win if the vapor pressure can not be attained (e.g., the vapor is being pumped away) and condensation will win if the vapor pressure is higher than what it should be at T. The atmospheric pressure boiling temperature of a liquid is the temperature at which the vapor pressure equals the ambient atmospheric pressure. With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure thereby forming bubbles.
Vapor Pressure of a Liquid http://www.chem.purdue.edu/gchelp/liquids/vpress.html
Vapor Pressure of a Solid The vapor pressure of a solid is also caused by the fact that solids also have a tendency to evaporate (to sublimate) and the corresponding gas (vapor) has a tendency to condense back to the solid form. These trends come into equilibrium in a closed system, which is at some temperature T, thereby establishing the solid s vapor pressure at the temperature T. When this vapor pressure is significant, the solid is said to sublimate
Vapor Pressure of a Solid http://www.chem.purdue.edu/gchelp/liquids/vpress.html
Vapor Pressure of Water At Various Temperatures Vapor Pressure of Some Other Liquids at Room Temperature (20C) Temp C Vapor Pressure (Torr) Liquid Vapor Pressure (Torr) 100 760 Benzene 74 50 93 Ethyl Alcohol 43.9 25 24 Methyl Alcohol 96.0 0 4.8 Acetone 184-40 0.1 Turpentine 4.4-78.5 5x10-4 Water 17.5-196 10-24 High Vacuum Pump Oil 10-7
System Leak Checks A leak can be determined by performing an up-leak test: A process chamber is pumped down to a desired base pressure The chamber is isolated from the vacuum pumps and monitored for a rise in pressure The degree to which the pressure rises determines the type of leak
Real Leaks Actual rate of rise depends on size of leak Time Public Domain: Generated by CNEU Staff for free use
Virtual Leaks Rate of Rise Curves (Outgasing) Time Public Domain: Generated by CNEU Staff for free use
Combination of Real and Virtual Leaks Virtual leak source is depleted: real leak takes over Time Public Domain: Generated by CNEU Staff for free use
Unit 2 Infrastructure Lecture 6 More on Vacuum Systems, Part III
SUMMARY More on Vacuum Part III Overview of Vacuum Systems Vacuum pumps Vacuum leaks