A probe is set to receive a thin film of Aluminum via a thermal evaporation vacuum system. The reason to coat the fiber in the vacuum system is to create a clean surrounding for the coating process. The thermal evaporation vacuum system includes a main chamber, ion gauge, roughing valve, high vacuum valve, fore-line valve, diffusion pump, mechanical pump, flow switch, heat sink, solid state relay, neutral bus, solenoid valves, and ammeter.
It is highly necessary to maintain an interlock for the diffusion pump by using the flow switch and solid state relay. The interlock is needed to prevent damage on the diffusion pump. Specifically, if there is not enough water passing through the flow switch, the diffusion pump cannot be turned on. The diffusion pump needs water when pumping vapor cycle and gas flow inside the system. The diffusion pump, VHS-6, is operated at 120 Volts with the solid state relay controlled by a 24 Volts power supply. Therefore, a solid state relay is designed to switch the power for the load circuitry of the diffusion pump. The neutral bus is technically allowed to bond the wires of the system. This thermal evaporation vacuum system is used to coat a thin layer of Aluminum on an optical fiber. Everywhere except the tip of the optical fiber has been covered in a thin layer of Aluminum.
The first step is to bring the system to a rough vacuum by using the rotary vane oil-sealed mechanical pump. The mechanical pump compresses the gases to a point slightly above atmospheric pressure by removing gases from the system. When the operation pressure is reached around 1*10 -3 Torr, the silicon oil vapor diffusion pump can be turned on. The silicon oil vapor diffusion pump cannot successfully operate at atmospheric pressure. The reason to choose silicon oil for the diffusion pump is the silicon oil is inert; the oil has no reactive with anything. When the metal coating process is finished, the entire evaporation vacuum system is locked up, and pressure inside the main chamber should be released to atmosphere. Therefore, the coated fiber inside the main chamber can be removed.
The process of operating the thermal evaporation vacuum system:
Always check that the air intake valve is closed before turning on the mechanical pump. If the air intake valve is opened, the Mechanical Pump will be operated in the atmosphere, which could not bring the system to rough vacuum level (759 to 1*10 -3 Torr). All valves must be in the closed position when turning on the mechanical pump. After ten seconds of turning on the mechanical pump, the foreline valve (valve 1) can be flipped to the open position. This allows the foreline to reach a rough vacuum. Wait 20 to 30 minutes for the foreline to pump down, then, close valve 1 and open valve 2. Only after valve 2 has been opened and it has been verified that the water is turned on, it is safe to operate the diffusion pump. If there is no water flow into the flow switch, which electrically connected to the solid state relay, then the solid state relay cannot be turned on. Inside the diffusion pump there is a heating element that heats the silicon oil to a high temperature; therefore the diffusion pump needs cooling. Without water-cooling, the diffusion pump will quickly become damaged and nonfunctional. To prevent this, there is a flow switch on the diffusion pump’s water supply line which controls the state of the solid state relay. Wait for 5 minutes after opening valve 2 before turning on water to operate the flow switch. This 5 minutes allows the mechanical pump to pump down on the cavity of the diffusion pump, i.e. allowing the diffusion pump to get down to an allowable operating pressure. Once the water is on, check that the exhaust water is running back to the sink. The diffusion pump cannot be operated at or exhaust (vent) to atmospheric conditions; therefore, it vents to the mechanical pump through valve 2. The diffusion pump should only be operated at pressures below 1*10 -3 Torr.
The diffusion pump must pump down for another 30 minutes before it can be exposed to the bell jar. Opening valve 3 allows the diffusion pump to connect and pump on the main chamber, which is known as a bell jar. Before valve 3 is opened, the system pressures must be equalized by cycling between closing valve 2 and opening valve 1 in an alternating manner. This allows pressures between the foreline, the rough line and the bell jar to be equalized. The foreline pressure must be monitored and toggle between the bell jar and diffusion pump a few times at this stage. This cycling process takes between 5 and 10 minutes.
To operate the diffusion pump, valve 1 is closed while both valve 3 and valve 2 are open. This creates a clear channel from the bell jar through the diffusion pump to the rough line, which allows the bell jar evacuate. Operate the diffusion pump for 30 minutes before checking ion gauge, which is connected to the main chamber. The ion gauge induces an ion current, which is amplified and is proportional to the pressure inside the main chamber. The button labeled ion is pressed to take a reading from the ion gauge; an idea pressure of 6.8 to 1.8*10 -5 millitorr should be read. 20 volts is applied with a variac transformer to the molybdenum melting boat. At 20 volts, the aluminum pellets in the melting boat will melt but not vaporize. In order to vaporize the aluminum, a voltage of 25 volts needs to be applied. The vaporized aluminum now coats the fibers because of the high vacuum and the relatively large mean free path. The whole vaporization and coating procedure takes approximately 5 minutes. After the coating procedure, the voltage from the variac transformer is reduced to zero.
When vaporizing the aluminum, the ejected aluminum will interact with the surrounding atmosphere, resulting in the formation of impurities affecting conductivity and the flow of current through the boat. If the ion gauge shows codes 2, then the lamp goes off. It means that that the pressure inside he bell jar is too low or the ion gauge is not connected.
Wait for 10 minutes to get the correct pressure.
To turn off the thermal evaporation vacuum system:
The Ion-gauge monitor is turned off, discharge Valve 3 and then close valve 3. This is to isolate the pressure in the main chamber from the diffusion pump. Power down the diffusion pump, but let the water keep running to further cool down the diffusion pump. Wait for 20 minutes before shutting down the Mechanical pump then turn off the tap water. Release the pressure of the bell jar by turning the air intake valve until hearing a strong hissing sound of the airflow. Once the airflow has stopped, the air intake valve is closed. The bell jar can be opened and raised to take out the coated tips. The coated tips will be mounted on tuning forks in order to be used as near field probes.
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