| PLD, clean bench, hot plate & air oven |
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SOP FOR PULSED LASER DEPOSITION (PLD) UNIT LOCATION: WL 105/106 In-Charge: Prof. Ashish Garg Super-user : Amritendu Roy, Tapendu Mandal User : Vikarm Verma, Somdutta Mukherjee, Anup Singh, Shailendra Gupta
· STAND BY CONDITION
o Pump Vented o Chamber at 10-1 mbar o GATE valve closed o VENTE valve closed o PROCESS VALVE closed.
· PROCEDURES FOR DEPOSITION
I. WITHDRAWAL OF THE SUBSTRATE HOLDER FROM THE DEPOSITION CHAMBER
PROCEDURE:
· Open the VENT(this is the “pull to vent” valve) valve just above the GATE valve. · Let the pressure of the chamber reach atmospheric pressure (1000 mbar). The pressure will be shown on CH-I gauge on the panel. (This pressure reading may not be accurate as the gauge does not have sensitivity at this pressure. Once the chamber vents the vent valve will be very easy to pull and no hissing sound of air getting in will be heard) · As soon as the chamber pressure reaches atmospheric pressure the chamber will open. Ensure the shutter on the substrate is at locked condition i.e., the shutter should be in front of the substrate to avoid any contact of the shutter with the chamber wall during the withdrawal of the substrate holder from the chamber. · Take out the substrate holder from the chamber.
II. LOADING THE TARGET AND THE SUBSTRATE
PROCEDURE:
· Remove the cover in front of the target holder. (this is called contamination shield and prevents one material getting deposited on near by target) · Take out the chosen target holder. · Fix the target at the center of the chosen target with silver paste.[†] · Fix the target holder back in its position (after proper drying of the silver paste) · Adjust the distance between the surface of the target and the outer surface (opening surface) of the chamber so that the intermediate distance is about 230 mm. (this defines the target plane) · Fix the target cover over the target holder. Ensure the distance between the surface of the target and the target cover is 2-3 mm. Double check it by rotating the target holder. Target should not touch the target cover under any circumstance. · Fix the substrate at the center of the substrate holder (using silver paste/clamp) . (Using clip is an option for samples where you do not want to have silver paste at all. But this does not assure you good thermal contact and hence the temperature can be as low as 100C from the heater temperature. When silver paste is to be used, the silver paste should be diluted with N butyl acetate. How much one has to thin it will come with practice. A good thumb rule would be to have the thinner twice in volume of the silver paste. Mix well this solution before applying on the sample holder. Be liberal in applying. Basically when one place the substrate on this solution and press on the substrate to squeeze out the excess silver paste solution from below the substrate the solution should come out from all sides. This makes sure that there is no excess solvent below the substrate and no air is trapped below the substrate. Leave the substrate this way for about 5 min for natural drying of silver paste solution. Fter the five minutes slowly heat the sample to 70C. The slow heating is to make sure that any excess solution below the substrate does not suddenly boils off and the substrate is not lifted off the heater. Leave at 70C for another 5 min before loading the heater flange.)
III. ALIGNMENT OF LASER
PROCEDURE:
· Switch on the laser[‡]. · Open the shutter of the laser. · Wear gogglesfor safety. · Open the nitrogen cylinder and maintain a pressure of about 1.2 bar. (adjust the flow of N2 to be about 20 SCCM as per the flow meter fixed near the cylinder) · Run laser. · Check laser spot on external blue ring, intermediate lens, glass window of the PLD unit. Laser spot should be formed at the center of all the above mentioned objects. (This is not a rigid requirement. These parts are much bigger than the laser beam coming out of the aperture also once aligned one need to realign it unless some one has interfered with the original alignment) · If the laser spot is not at the center of the intermediate lens adjust X-Y screw attached with it and fix the position. (the X-Y screws are to be adjusted to make the beam fall at the correct spot on the target and not for centering on lens) · Ensure the laser spot hits just above the center of the target surface. · Adjust Z-alignment of the intermediate lens to form a 3 mm long spot at the target surface. The spot can be collected at a fax paper and can be measured (A good image is when the spot on the target is a miniature version of the aperture. Rememebr that the beam falling on the target at about 45 degrees will make the beam broader width wise compared to the height of the beam) · Measure laser energy inside the chamber. (This should be at least done every 2 to 3 depositions and will indicate when the window on the chamber is to be cleaned) · Note down the initial angle and start and end angle of the target up to which laser can hit the target and not hit the target cover. (Do not go to extremes and always keep some safe margin. Also this is required only if you want to raster the target. For rotation only one needs to keep the start angle and end angle same.)
IV. CLOSING THE CHAMBER
PROCEDURE:
· Keep the substrate cover (shutter) in front of the substrate. · Place the gasket properly in the channel. · Close the chamber and ensure the permanent marks are matching. The heater should always be placed vertically. · At this condition the chamber is at 1 atm. pressure and the pump is vented and vent valve is closed.
V. EVACUATING THE CHAMBER
PROCEDURE:
· Open the vent valve below the gate valve to ensure the pump is vented and close it. · Open the GATE valve completely. · Switch on the rotary pump. When the pressure reaches ≤ 5×10-2 mbar, switch on the turbo pump. Pressure can be viewed on the CH-I pressure gauge. Once the turbo is stabilized put the turbo on “STAND BY OFF” mode to accelerate the evaculation. · Wait until the required base pressure is attained. (Typically 2×10-6 mbar)
VI . DEPOSITION
PROCEDURE
· Set the deposition temperature in the heater and wait for the process temperature to be reached. · Put the turbo pump in the STAND BY ON mode. · Ensure the process gas (oxygen) cylinder is open. · Close the gate valve up to a position where one feels some pressure on the wrist. · Open the process gas valve (green valve on the PLD unit). The pressure will suddenly increase and will come back to better vacuum (this is because the line from the gas valve to the flow meter is pumped down) · Put the 3-way valve in the control mode. (In this mode gas passes through the flow meter) · Adjust the flow rate (in the mass flow meter) and the gate valve position until the required process pressure is reached. (fine adjustment involves adjusting the gate valve and gas flow) · Set the parameters on the target carrousel controller. (make sure that the laser is on external trigger mode and is in ready position waiting for trigger pulses from the target carrousel controller) · Do pre ablation (at least 1 min) and open the shutter for the complete deposition. · At the end of the deposition choose the following options depending on process.
a) Close gate valve, close the substrate shutter, and vent the chamber to about 500 torr with process gas, switch off heater and wait for the temperature to reach close to room temperature. Vent the chamber with either process gas or with air to remove the sample. b) Keep the process conditions as it is and switch of the heater and wait for the temperature to reach close to room temperature ( this is sometimes required when one has to cool the sample under process conditions). Close the gate valve and vent the chamber with either process gas or with air to remove the sample. c) Close the gate valve and vent the chamber with process gas up to about 500 torr and switch off heater after required time (this is annealing in process gas). Vent with either process gas or with air to remove the sample once the sample has reached close to room temperature. d) Close the gas valve, open the gate valve fully and put the pump on full speed. (this is for vacuum annealing of the sample after deposition). Switch off heater after required time, vent with either process gas or with air to remove the sample once the sample is close to room temperature.
Once the sample is removed (it is always a good practice to clean the heater surface of remaining silver paste by sanding it with a fine emery paper after the excess silver paste is removed with the blade. Remove the debris by a small vacuum cleaner. This also is a help to the next user) it is advisable to keep the system in vacuum before leaving the lab for the day.
Open the gate valve fully and start the rotary pump. When the pressure reaches 5×10-2 mbar, start turbo. Wait until the vacuum reaches about 10-4 mbar close the gate valve and switch off the turbo and rotary. Switch off all electrical
You are now ready to leave for home/hostel.
Important: always make it a practice to be around the system when the laser is firing observing the plume from side port, top port etc . This gives a good experience to identify if anything is wrong when the system is used later. Also make sure that the laser plume is centered on the substrate. If this does not happen you know where to mount the sample in the next deposition. Since the sample is small and heater is big one can always mount the sample away from the center of the heater to get the plume fall on the sample.
Also keep watching the pressure gauge, flow meter etc to make sure that the flow, pressure etc are what they should be. If not make fine adjustments to get to the required values of the process parameters.
Best of Luck
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