This experiment consists of two parts. In the first, we will use an evaporator station to produce a thin metallic film. In the second part, we will measure the resistance of this film as a function of temperature.
WEEK I: THE EVAPORATOR
The evaporator is a standard component of a physics laboratory, and contains a high-vacuum pumping station, the operation of which is a standard skill for experimental physicists the world over. The reason for the high-vacuum system in this experiment is to allow a clear path between the source material to be evaporated and its final destination. For some experiments, the high vacuum also serves to prevent oxygen from the atmosphere from degrading the material to be evaporated.
Do a quick literature search to familiarize yourself with the design and operation of a diffusion-pump based vacuum station. Compare any diagrams you find to the system that we have in the lab.
Our vacuum system consists of a vacuum chamber (a "bell jar") connected through pipes and valves to two very different pumps: a mechanical "roughing" pump and a diffusion pump. The diffusion pump can pump to a much better vacuum than the roughing pump, but cannot move large volumes of air, while the roughing pump can move large volumes of air, but cannot reach a very good vacuum. Furthermore, the diffusion pump, in operation, needs to have a second pump, in this case the roughing pump, pumping on its exhaust in order to remove the gas it has pumped from the system. Thus, the system of hoses, tubes, valves, and pressure meters serves to allow us to pump out the bell jar with the roughing pump, turn on the diffusion pump and allow it to warm up while pumping on it with the roughing pump, and then open up the bell jar to the diffusion pump to reach our ultimate vacuum.
Operating the system requires always having the right valves open, and being aware of the pressure in various parts of the system. It is a lot like learning to drive a car with "standard" gearshift (rather than "automatic" transmission). If you are ever in doubt about what to do next, please ASK! We do not wish to have to do the equivalent of replacing the transmission of our system. (although such an exercise is also very educational)
Here, in short, is the standard procedure for operating the system.
A. STARTING THE DIFFUSION PUMP
1. Ensure that the diffusion pump is isolated from the vacuum chamber
(Gate valve closed) and from the roughing pump [RP].
2. Ensure that the roughing pump is isolated from the vacuum chamber and the diffusion pump [DP]. Turn on the roughing pump, and allow it a minute or two to quiet down, if necessary. (The sound (and smoke) of the roughing pump pumping on atmosphere is quite different from the sound when pumping on [near] vacuum. Look, listen, and note the difference.)
3. Open up the valve between roughing pump and diffusion pump to start pumping on the DP's exhaust. You may want to turn on thermocouple 2 ("TC2") to check that this is significantly lower than atmosphere.
4. Turn on the cooling water to the diffusion pump. There should be a steady stream of water flow. Later, when the DP is on and warm, you may want to increase the flow so that the copper water flow pipe is warm, not hot, to the touch.
5. It is now safe to turn on the diffusion pump. Give it at least 15 minutes before checking whether it is pumping.
B. PREPARING THE VACUUM CHAMBER (Steps 1-4 can be done concurrently
1. Clean the bell jar, if it needs it.
2. Mount a filament, with the material to be evaporated, between the two electrodes in the vacuum chamber.
3. Mount a substrate (the object onto which you evaporate your material) above the filament
4. Optional: You can mount the "chimney" around the filament to limit the spray of material onto the bell jar, and limit heating of the substrate. You can also place a movable shutter between the filament and the substrate, to limit the amount of material deposited.
NOTE: Some materials do not belong in a vacuum system, ever. Blocks of wood, or other porous materials, take forever to yield up the air hidden in their pores. Water will boil under low pressure, even at room temperature, and the water vapor gets into the vacuum oil (for either the roughing pump or the diffusion pump), limiting its efficiency.
5. When you are ready, you can begin pumping down on the vacuum chamber. First check that the air inlet valve is closed.
6. If the roughing pump is not currently in use (otherwise read Step 7), make sure that the valve between it and the diffusion pump is closed, then slowly open the valve between RP and the vacuum chamber. There should be a lot of smoke coming out of the RP's exhaust. Make sure that it is vented to the outside of the building, but do not open the valve fully until the smoke clears. You may want to push down on the bell jar for a minute if it is not making a good seal with the vacuum chamber, which would prevent the system from pumping down.
7. If the RP is in use, we will borrow it for a few minutes from the DP. It is important to remember to continue pumping on the DP after we are done using the RP to pump down the vacuum chamber. Simply perform the actions in Step 6, remembering to leave DP isolated from the roughing pump while performing them.
8. Monitor TC1, which shows the pressure in the vacuum line between the vacuum chamber and the roughing pump. When it reaches 0.05Torr ( equals 0.05mm of mercury, or less than 1/10,000 an atmosphere), you are ready to pump on the vacuum chamber with the DP. Close the valve between RP and vacuum chamber. The pressure at TC1 should stay reasonably stable. If not, ask your instructor for HELP. Next, open the gate valve which allows the DP to pump on the vacuum system. This is a many-turn valve, which does not begin to open immediately. (Take advantage of down time during week two to check how the valve really works.) Keep one eye pinned on TC1. When the gate valve finally opens, the needle on the TC1 dial should quickly move all the way to the right side of the dial. Finish opening the valve up all the way. It should take five minutes or more before the vacuum is appropriate for you to do your evaporating.
EVAPORATING THE FILM:
Your instructor will show you. (The detailed instructions above are included because of the importance and universality of the steps for vacuum systems around the planet. You may need to refer to them again years down the road.)