Vacuum Pumping process for Large Camera Dewars supplied by PixelVision of Oregon

Transcription

1 Vacuum Pumping process for Large Camera Dewars supplied by PixelVision of Oregon MacOS9.2.1: :Eudora Folder:Attachments Folder:Process for vacuum pumping larg Edited: :56 1 of 5

2 Process for vacuum pumping large camera Dewars. Dewars continually leak some amount of air and water vapor. While LN 2 is present in the Dewar LN 2 tank, water vapor, and some of the gasses are condensed or frozen on the surface of the LN 2 tank. At some time that gas and water will need to be pumped out of the Dewar so that the pumping capacity of the LN 2 tank is not exceeded. Apparatus: A rotary vane vacuum pump (oil sealed). A foreline trap to prevent back streaming of pump oil into the Dewar. A vacuum gauge for the foreline. A vacuum line to connect the pump to the Dewar Fittings that allows introduction of N 2 into the vacuum line. A pressure regulator for the N 2. A vacuum pump port valve actuator for the Dewar pump port. A source of dry gas such as N 2 from the LN 2 Dewar used to fill the Dewar with cryogen Process for vacuum pumping the Dewar. 1. Connect the pump and N 2 source to the Dewar but keep the Dewar pump port closed. 2. Start the flow of N 2 then start the pump and establish a pressure of 5 to 15 Torr in the foreline by adjusting the regulator knob. Open the GasBalast Valve on the pump. 3. Open the pump port slowly to avoid rapid filling of the Dewar. Pump the water vapor out of the Dewar while keeping the Dewar pressure between 5 and 15 Torr. This avoids permitting the water vapor to enter the solid (ice) form, thus lowering its vapor pressure which decreases its pumpability. Close the Pump Port Actuator valve at the Dewar after the period for pumping water vapor out of the Dewar has been finished. 4. Restore the foreline trap by baking before lowering the pressure in the Dewar below 5 Torr. Close the Pump Port Actuator valve at the Dewar. Close the GasBalast Valve. Turn on the ForeLine Trap heater. After the trap has reached operating temperature, close the N 2 source valve at the vacuum line. Pump on the closed vacuum line and foreline trap for hrs to restore the foreline trap. Turn off the power to the foreline trap and allow it to cool. Open the Pump Port Actuator valve at the Dewar. (Do not allow the pressure in the vacuum line to be reduced until the foreline trap has been restored. This is necessary to keep oil out of the vacuum line). 5. Establish the working vacuum in the Dewar. This is at the pump base pressure of about 5 to 10 mtorr and must be done after the foreline trap has been restored. Open the Pump Port Actuator valve at the Dewar. 6. Close the pump port, turn off and remove the pump, pump port actuator and vacuum line from the Dewar. Leave the pump open to ambient air. 7. Fill the Dewar with LN 2 and wait for cool down. 8. Use the Dewar until the internal pressure has increased by 150mTorr. Keep the Dewar filled with LN 2 while it is in use. 9. Before bringing the Dewar up to ambient temperature, set the CCD temperature control to 325K. This keeps the CCD s warmer than the LN 2 tank so that the condensed water vapor stays on the tank. When the LN 2 is completely empty the CCD temperature will be about 320K. 10. Allow the LN 2 to boil off without refilling. 11. Reconnect the pump and N 2 source starting at Step 1. Repeat these steps as necessary. Discussion MacOS9.2.1: :Eudora Folder:Attachments Folder:Process for vacuum pumping larg Edited: :56 2 of 5

3 The triple point of water occurs at 4.6 Torr and 273K. If pressure is the independent variable the temperature will seek the boundary between two forms of the substance for that pressure. If the pressure is below the triple point the water will be on the boundary between ice and vapor. If the pressure is above the triple point the water will be on the boundary between liquid and vapor. Ice is to be avoided as the vapor pressure of ice is much lower than that of liquid water. Thus a pressure of about 10 Torr is chosen. The pressure should be less than 19 Torr so that the vacuum pump can pump the water vapor out of its oil. The foreline trap prevents oil back streaming that increases as the pump reaches its vapor pressure. At pressures of 5 Torr there is essentially no back streaming. The pump oil has a vapor pressure of 1E-6 Torr. The foreline trap is restored when it is heated and the pump is at its base pressure. This is done after the water vapor has been removed from the Dewar. Restoration takes several hours (16hrs). The trap needs to be restored before each use because it is stored at atmospheric ambient. Graph of the triple point of Water Pressure Solid Liquid Vapor Temperature The choice of the vacuum pump becomes one of choosing a pump that will work at 10 Torr as well as one that has an adequately low base pressure. The Rotary Vane Pump is appropriate for this service when used with a foreline trap. The turbo molecular pump is not suitable since it must transition through the high-pressure region rapidly to avoid overheating. The vacuum pump can pump water vapor but must have the GasBallast valve open. The water vapor tolerance is 18 mtorr before the oil saturates with water. Therefore when pumping the water vapor out of the Dewar, operate with the GasBallast valve open, and with a pressure between 5 Torr and 15 Torr. After pumping the water vapor for a period of time at a pressure from 5 to 15 Torr, the operating vacuum may be established in the Dewar. To prepare to pump the Dewar to the pressure limited by the pump s capability: Close the Pump Port Actuator valve at the Dewar. Close the GasBalast Valve. Turn on the ForeLine Trap heater. After the trap has reached operating temperature, close the N 2 source valve at the vacuum line. Pump on the closed vacuum line and foreline trap for hrs to restore the foreline trap. Turn off the power to the foreline trap and allow it to cool. Open the Pump Port Actuator valve at the Dewar. (Do not allow the pressure in the vacuum line to be reduced until the foreline trap is up to temperature as a means to keep oil out of the vacuum line). MacOS9.2.1: :Eudora Folder:Attachments Folder:Process for vacuum pumping larg Edited: :56 3 of 5

4 Additional information copied from the Krut J. Lesker website ( Traps & Filters: Technical Notes Foreline Traps Foreline traps operate between a 'gas transfer' pump and a mechanical pump in a high-vacuum system. Of the three distinct types of foreline traps, the most common type stops oil vapor migration from the mechanical pump to the highvacuum pump. The second type, increasingly popular in the semiconductor industry, protects the mechanical pump from dust and particles generated by vacuum systems. The third type traps condensable vapors (such as water) as they are pumped from the chamber. Trapping Oil from Mechanical Pumps. Three trap designs work to trap oil from mechanical pumps: Micromaze Molecular Sieve Assimilation The first two designs trap large or polar molecules in the pores of a material with an extremely large surface area. The Micromaze trap uses a proprietary porous sheet material exclusive to the Kurt J. Lesker Company. Molecular sieve traps use a pelletized zeolite - a synthetic molecular structure with holes in the crystal lattice. The term conversion trap once described devices filled only with copper 'wool' elements; now, however, we offer activated alumina and carbon filters for these traps. Trapping Dust from the System. The designs for trapping dust from a system include the standard cyclone trap and the dust (or inlet) filter. In the cyclone trap, dust-laden gas enters a cylindrical chamber tangentially. The gas pumps from the center of the chamber, while the dust (by momentum) continues to swirl around the chamber walls, eventually falling into a collector. The dust filter uses a fibrous element that must be replaced at regular intervals to avoid loss of gas conduction. Trapping Vapors from the System. All large quantities of condensable vapor should be trapped before the mechanical pump, since even an innocuous vapor can cause unacceptably high foreline pressures. Mechanical pump vanes compress the exhaust gas to slightly higher than atmospheric pressure, and vapors condense rather than exit the pump as gas. But as the vanes pass the exhaust and connect the pump to the low pressure inlet, the liquid re-evaporates, causing high 'un-pumpable' pressures in the foreline. To remedy this use a liquid nitrogen (LN2) foreline trap. Most vapor traps using liquid nitrogen (LN2) require rigorous maintenance. Any decrease in LN2 level may cause a vapor slug to enter the pump. On system shutdown, the LN2 must be vented to avoid a pressure explosion, or even a real explosion if using a flammable vapor. A Note on Oil Backstreaming. Using foreline traps to stop backstreaming has well established benefits. But many people misunderstand the mechanics of effective trapping. Oil in the foreline close to the mechanical pump is a mixture of normal-size and 'cracked' (fragmented) molecules. Without a foreline trap, these molecules migrate into the high vacuum pump by a combination of wall 'creep' and vapor transport. Neither diffusion pumps nor turbomolecular pumps provide effective barriers to the vapor. The diffusion pump fluid has a higher boiling point than the mechanical pump oil. If mechanical pump oil gets into the diffusion pump, it will backstream rapidly into the system. The spinning rotors of a turbomolecular pump will block vapor migration into the system, but a foreline trap is still needed to prevent oil vapor from condensing on low-level rotors when the pump is not running, since condensed oil can then migrate to higher levels by evaporation-condensation every time the pump stops running. Indeed, frequent pump shutdowns (using the chamber up-to-air valve to quickly stop the rotor) raise the rotor temperature well above ambient, promoting oil evaporation and transfer throughout the pump. The only way to prevent oil vapor from reaching the chamber is to install a good foreline trap and then rigorously maintain it by frequent baking (150ºC to 300ºC) to prevent oil saturation. Bakeout requires a high-quality shutoff valve between trap and chamber -- one capable of being heated to stop oil vapors from condensing on its cold surfaces. In superior system designs, foreline traps have valves on both sides. When the pumps are switched off, both valves shut to prevent water adsorption from the atmosphere. (Even with anti-suckback valves, all mechanical pump inlets should be opened to the atmosphere when the pump is off.) Replace sealed assimilation traps before oil vapor breakthrough occurs. Rechargeable assimilation traps are maintained by replacing the active element at regular intervals. The time between trap or element replacements depends on ambient temperature, distance between trap and pump, type of pump, type of oil, etc. For typical applications, we recommend replacement every few months. MacOS9.2.1: :Eudora Folder:Attachments Folder:Process for vacuum pumping larg Edited: :56 4 of 5

5 LN2 Traps Liquid nitrogen (LN2) traps operate between diffusion pumps and vacuum chambers to prevent backstreaming oil vapors from reaching the chamber. These traps can also lower the partial pressure of water vapor in the chamber. Typically, they mount directly on the top flange of the diffusion pump. We do not recommend LN2 traps for systems containing large quantities of condensable vapor, which should be trapped in an LN2 foreline trap (see foreline traps above) so that the condensate can be removed and dumped. Most turbomolecular pump manufacturers recommend against placing an LN2 trap above the turbo inlet. They explain that water vapor frozen on the trap will drip into the pump as the trap thaws. However, that the system has enough water vapor to form drops, strongly suggests that it needs an LN2 trap. LN2 traps require simple maintenance. Simply keep the LN2 level at a constant level since variations will cause some gases to evaporate as the trapping surfaces warm. For this, we recommend LN2 Level Controlling Devices. Well-designed LN2 traps can greatly enhance the ultimate pressure of a system. Take our VZCCT traps as an example. They can work with diffusion pumps (supplemented by Ti Sublimation pumps) to achieve pressures in the mid torr range. Oil Mist Eliminators Avoid breathing the exhaust vapors and aerosols produced when pumps displace large volumes of gas. Oil mist eliminators work especially to stop these aerosols. Typically, the eliminators contain a metal or non-organic wool making the gas's path too torturous for the heavy aerosol particles to follow. More recent designs use electrostatic fields to destroy the droplets. Oil mist eliminators generally attach to a pump exhaust port, allowing captured oil mists to drain back into the pump. We cannot overemphasize the hazardous nature of gases from many semiconductor process systems. Oil mist eliminators will not trap these gases. They must be properly filtered and vented into the outside air. However, mist eliminators reduce the quantity of oil released into the environment and should be used on all mechanical pump installations. Baffles A baffle operates between a diffusion pump and either its LN2 trap or the chamber. The baffle vanes form a chevron or venetian blind pattern obscuring the line of sight. When used with a trap, the baffle serves as a thermal barrier between the radiating diffusion pump and the trap. In the absence of a trap, the baffle makes a lower-cost, but less effective, substitute for LN2. (Note: Better LN2 traps have integral water-cooled baffles.) A liquid cools the baffle by circulating either through the baffle vanes or through an attached heat exchanger. Watercooled baffles stop much of a pump's backstreaming, particularly when used with the lowest vapor pressure diffusion pump oils, such as Santovac 5 and DC705. Other baffles use cryogenic liquids, such as an acetone/co 2 slush. Filters Oil Filters. On oil filter attaches to a mechanical pump. It consists of one or more filter elements housed in separate canisters and interconnected by pipes mounted on a baseplate. An electrically driven pump forces oil from the mechanical pump body through the elements and back. The oil filter serves several purposes. Primarily, it removes particulates form the oil to reduce wear on close-tolerance pump surfaces. Typically, the filter elements trap particle sizes greater than 0.2 microns. Second, with special elements, the filter neutralizes hydrous or Lewis acids in the oil. Third, the correct filter elements remove water from the oil. As an incidental benefit, the filter provides cooling by circulating the oil for several seconds outside the body of the mechanical pump. Inlet Filters. Inlet filters are simply an alternative name for the dust filters described above, under 'Foreline Traps'. MacOS9.2.1: :Eudora Folder:Attachments Folder:Process for vacuum pumping larg Edited: :56 5 of 5