Chico PHYS 427 - Vacuum Techniques and Thin Film Deposition Experiment 3

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California Institute of Technology Physics 77Vacuum Techniques and Thin Film DepositionExperiment 3(October 2001)1IntroductionMuch of modern experimental physics is done under vacuum. Design and construction ofvacuum apparatus is one of the most useful ”bread and butter” skills an experimentalist incondensed matter, atomic, or optical physics can have, and the subject of vacuum engineeringis a vast one. This lab serves as an introduction to basic vacuum techniques and thin filmgrowth, another often essential skill for condensed matter physicists. This lab is an optionalprerequisite for Experiment 10, Condensed Matter Physics at Cryogenic Temperatures, forwhich you can grow your own samples for Weak Localization measurements if you choose.2PressureandgasflowIn vacuum work, pressures are almost always measured in millimeters of mercury, or torr.One torr is just the pressure necessary to support a column of mercury with a height of onemillimeter. The conversion to units more familiar to readers of physics textbooks is1atmosphere = 101kPa = 760torrThere are two pressure regimes of interest to the scientist working with vacuum systems,and gases behave differently in each regime. The first, the viscous flow regime, describes thecase where gas flows as a fluid, where the mean free path of the gas molecules is much smallerthan the dimensions of the apparatus. The second, the molecular flow regime,describesthe high-vacuum case, where the mean free path is much longer than the characteristicdimensions of the apparatus. In this regime, gas molecules interact almost entirely with thewalls of the chamber, acting independent of each other.Gas flow in either regime is measured in torr liters per second,whichisequivalenttomass per second. The conductance of a tube describes how much gas flows through the tubefor a given pressure differential between the ends. If Q is the m ass flow, P1is the pressureat the input of the tube, and P2is the pressure at the output, then the mass flow is given byQ =(P1− P2)C1where C is the conductance of the tube. Conductance in the viscous flow regime is propor-tional to the average pressure in the tube and is quite high, compared to the molecular-flowregime, because the gas molecules push each other along. In the molecular-flow regime,conductance through a tube is independent of pressure and is given byC =12literssecond!D1cm"3!1cmL"where D is the diameter of the tube in centimeters, and L is its length, also in centimenters.Pumping speed is expressed in liters per second. The amount of mass going through thepump is given byQ = PSpwhere P is the pressure at the inlet of the pump, and Spis the pump speed. It is not hardto show that the net speed of a pump connected to a vacuum chamber by a tube is1S=1Sp+1C(1)and that the time required to pump the system from an initial pressure of P0down to P ist =2.3VSlnP0P(2)where V is the volume of the chamber.3VacuumPumpsA large number of clever designs for vacuum pumps have been implemented over the years,dating back to the first leather-and-grease sealed, hand-operated pumps of the 1600’s. Thesefirst pumps were modified ships’ water pumps, used for pulling water out of the holds ofthe sailing ships of the day, and they operated by a simple valve-and-piston mechanism.The valve-and-piston principle is still the most widely used way of extracting air in theviscous-flow regime, though today our implementation is considerable more efficient! Modernmechanical pumps feature multiple stages, specialized low-vapor-pressure oil sealants, andelectric motors. Good, modern mechanical pumps can often attain base pressures of a fewmillitorr or a few tens of millitor, though below about 100 mtorr the oil used in them willoften leak back into the chamber being pumped on. This is called backstreaming and isusually undesireable. Backstreaming can be eliminated by placing a trap or high-vacuumpump between the mechanical pump and the chamber.Mechanical pumps are seldom operated below 100mtorr, and for this reason they are oftenreferred to as roughing pumps.Toachieveevenamoderatevacuumof10−2torr or better,a different pump design must be employed. The most common and reliable high-vacuumpumps in use today are turbomolecular pumps, or turbo pumps for short. These are basicallyjust very high-speed fans, whose blades are moving at speeds comparable to the speeds of2Figure 1: Cross section of a single-stage, rotary-vane mechanical roughing pump. Gas ispulled in the inlet (arrow pointing down), circulated counterclockwise and compressed, thenblown out through a ball valve on the outlet. The theoretical ultimate base pressure is thepressure at the outlet (approximately atmospheric) divided by the compression ratio.gas molecules. Turbo pumps are capable of sustaining very high compression ratios,theratio of the gas pressure at the output to that at the input. Typical compression ratiosare on the order of 107for air, for an outlet pressure of 100mtorr. This low outlet pressureis maintained by a mechanical pump, which acts as both a roughing pump for the systemand a backing pump for the turbo. One advantage of using a turbo pump in conjunctionwith a mechanical pump is that the turbo pump’s compression ratio depends strongly on themolecular weight of the gas being pumped. Specifically, the log of the compression ratio isproportional to the square root of the molecular weight of the gas. Because the oils used inmechanical pumps typically have very high molecular weights, the compression ratio accrossthe turb o pump for these oils is considerably higher than 107,andtheturbopumpeffectivelyblocks any backstreaming from the roughing pump.Speeds for turbo pumps are usually independent of the type of gas being pumped. Turbopumps are specified by their speed, and the small turbo pump used in this lab has a speedof 80 l/s.4 Chambers and SealsTwo things that limit the level of vacuum in any experiment are leaks and outgassing. (Bothare mass flows and are expressed in torr liters per second.) Leaks are just poor seals thatallow air to enter the chamber from the outside atmosphere. Outgassing refers to sources ofgas ”stored up” inside the vacuum chamber and released slowly into the vacuum. Typicalsources of outgassing are trapped pockets of air in blind screw holes, rough surfaces, and3Figure 2: How a turbo pump works. The rotor spins fast enough to impart a significantdownward component to the velocity of the gas molecules, creating a pressure


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