Procedure for Laser Ablation of YBCO

 

(Rev 2 10/24/97)

O. General

  1. If you have any questions, ask me (Joe Hilliard). It is far better to ask about equipment or procedures you are not familiar with than to risk equipment damage or personal injury.
  2. The yellow safety goggles must be worn and the warning light by the door must be turned on whenever the laser is being fired.
  3. The excimer laser in this set-up is powerful enough to singe wood. Do not place anything in the beam path that does not belong there. Do not adjust the optics in the beam path while the laser is firing with the shutter open. Note that reflective objects, such as tools or wristwatches can reflect the beam in a random direction if placed in the beam path inadvertently.
  4. Cooling water MUST BE ON and flowing to the proper locations whenever the sample heater or the turbo pumps are in use.
  5. The high vacuum chamber and anything which will go into it must be clean and free of fingerprints because fingerprints and other contamination can outgas for a long time and prevent efficient pumping. Generally, this means you must wear clean PVC or latex gloves when working in the chamber or handling anything that will go into it.

 

I. Clean old substrates (if necessary)

  1. Mount substrate on polisher chuck using crystal bond and hot plate. Allow to cool.
  2. Select desired bowl and polishing slurry.
  3. Mount chuck in collar with set screw. Set up bowl and chuck in polisher.
  4. Polish until all old material is removed and a shiny finish is obtained.
  5. Remove chuck from collar. Reheat chuck to loosen crystal bond and remove substrate from chuck.
  6. Clean substrate in acetone, isopropyl, and methanol
  7. Clean chuck and collar. Use acetone to remove crystal bond, DI water to remove water based slurries, or isopropyl to remove oil based slurries.
  8. If a water based slurry was used (typically mastermet), clean the bowl thoroughly with water.
  9. Replace the polishing pad with another of the same kind if it is worn out. If we run out, pads, slurry, paste thinner etc. can be ordered from Buehler.

 

 

II. Sand YBCO targets

  1. For best results, YBCO targets should be cleaned by sanding before every ablation run. Silicon-carbide paper of 120 or 150 grit should be used. 220 grit can be used for a final fine sanding.
  2. Remove the targets by unscrewing them with your left hand while holding the mounting screw in your right hand, so that it won’t fall down into the chamber. Put both the mounting screws and the targets on a clean piece of filter paper or kimwipe. Make a note of where the targets were on the target holder so that they can be remount in the same locations.
  3. Sand the targets on the black silicon-carbide sandpaper until the target surfaces are a uniform black with all damage from the previous ablation sanded away.
  4. If a target comes off of it’s copper chuck, it can be reattached with silver paint in a similar manner to substrate mounting (described below).
  5. Blow the dust off the targets with a Chemtronics can or with nitrogen.
  6. Remount targets in the same locations on the target holder using the mounting screws you removed earlier.

 

 

III. Mount substrates on shuttle, load into chamber and pump down

  1. Attach substrate to shuttle with silver paint (Ted Pella Leibsleiter 2000), making sure that the paste-substrate interface is smooth and uniform so that thermal contact will be uniform. The edges of the sample can be patted down with q-tips and slightly moved from side to side to expedite smoothing of the paste.
  2. The shuttle with the samples on it should be left in the dessicator to dry for at least 4 hours, preferably overnight.
  3. Mount the deseried targets in the appropriate places. To do this, first place the 2-56 screw in the desired hole. Then use your left hand to center the 1 inch copper chuck on the screw and screw the target on while holding the screw in place with your right hand.
  4. Place the shuttle with the painted on substrates into the slot on the front of the heater assembly, making sure the thremocouple thermometer wire goes into the hole in the bottom of the shuttle.
  5. Close the door and turn the black roughing pump (switch on the floor). It is a good idea also to rough out the load lock (top) portion of the chamber, since this will make it easier to open the gate valve to backfill the chamber with nitrogen from the load lock later. (However, the roughing valves from the orange pump should not be open for the top portion of the large chamber and the small chamber on the side of the laser table unless both are alread at turbo pump vacuum.)
  6. When the pressure is down to around 0.1 torr,. Then turn on the main turbo pump (controller on the left). (If it takes too long to get to this pressure, check the iris in front of the turbo pump port. Set it to 152 to open it all the way if necessary.) The on switch should light up and the rotation indicator should start to increase to its max. If this doesn’t happen, you should be able to reset the turbo pump by holding down the reset switch.
  7. When the pressure reads near 1 x 10-3 on the thermocouple gauge, turn on the ion gauge. Pump down to the low 10-6 torr range. This should take of order an hour. You can pump longer to a lower pressure if you want to.

 

 

IV. Start up Excimer Laser

  1. Turn on the main power (big red plastic switch) and the control unit power (key switch). Wait 8 minutes for the initial timer to expire. The timer is there so that the (roughly $5000) hydrogen thyratron switch will have time to properly warm up before the laser is fired.
  2. After the timer expires, the control unit will read out the pressure in mbar, along with other information, such as the gas mixture selected. Test the maximum power per pulse as follows. (Leave the shutter closed for this whole step.) Fire the laser (run laser; execute)with the shutter closed at the rep rate you plan to use (typically 3 Hz) and at maximum high voltage (HV mode, 32.0 kV) until the power reading stabilizes. Record this as the maximum power. Stop firing the laser (break or stop). If you can’t get enough max. power, you will have to refill the laser gases
  3. Switch to (energy – no gas replacement mode) and set the desired energy per pulse. Due to an advertised feature (bug) in the software, the pulse energy cannot be set above 69% of the nominal maximum pulse energy for the selected gas mixture. If you want a higher pulse energy, you will have to obtain it yourself by going back to HV mode and adjusting the high voltage. Adjust the lens position to change the laser spot size on the target if desired. The pellet is in front of the focal spot of the lens, so moving the lens away shrinks the spot and moving it close enlarges the spot. Together the power per pulse and laser position determine the intensity of the laser spot.

 

 

V. Heat the substrates, wait for outgassing, then start the

  1. These steps can be interspersed with the steps in IV once you are familiar with everything.
  2. Make sure the shutter is in place to protect the YBCO targets from outgassing contamination coming from the substrates and silver paint as they are initiall heated up.
  3. Choose the desired set point on the temperature controller (typically 790 to 800 C) and press start to begin heating the substrates. It will typically take about 5 to 10 min to reach the desired temperature and stabilize. When you get there, the shuttle should be a uniform incandescent orange, indicating a uniform temperature. At this point, different kinds of substrates will be different colors. MgO will appear white, SrTiO3, will appear yellow-orange, and LaAlO3 will be a yellow color between these two cases.
  4. If part of a sample is significantly darker than the rest, this indicates poor thermal contact by the silver paint in the dark region. Depending on how important the dark region will be to your final product, you may want to abort the procedure at this point and start over with repainting the samples on the shuttle again when everything is back to room temperature.
  5. If the thermal contact is acceptable, allow the sample to outgass at least until the orignial desired base pressure is re-established. (I typically go down to 1.0 x 10-6 torr or lower, which takes over an hour.)
  6. If the MKS flow controller is not already on, turn it on and wait for it to warm up. It is warmed up when the flow reading is zero or close to it.
  7. Turn off the ion gauge. Turn on channel 3 to start the oxygen flow. Adjust the iris in front of the turbo pump port to obtain the desired equilibrium pressure of oxygen. typical values are 185 sccm for the oxygen flow, 351 for the iris setting and 100 mtorr for the eqm. pressure. The iris may need to be periodically adjusted to compensate for drift in the eqm. pressure.
  8. Often, a film of unknown origin (at least to me) is observed in the middle of the substrates after they are heated up. This film is most easily seen when the substrates are illuminated with a flashlight through the window. Fortunately, this film disappears from the outer edges inward once the oxygen is turned on. After typically 10 to 20 minutes with the oxygen on, the film will be gone and the substrate will appear uniform under flashlight illumination.

 

VI. Align the sample and pellet

  1. Set the laser rep rate to 1 Hz. Open the shutter.
  2. Move the substrate and pellet to roughly the correct location. (This will help you avoid non-YBCO ablation while you are aligning.) Typically this means setting them to the same locations as were used in the last ablation, assuming that the substrate and pellet are mounted on the same place on their respective holders. The substrate position is measured in mm (x,y,z) and is adjusted by stepper motors which respond to the computer and the joystick. The pellet position is measured in degrees and inches and is controlled by hand with the dials on the end of the pellet motion feedthrough. However, the pellet can also be scanned by stepper motors. You should zero the pellet position on the computer so that alignment can be done with the pellet in the center of its scan pattern.
  3. When you are ready, fire the laser for one (or a few) pulse(s) at a time and make sure the beam spot is hitting the proper target. Adjust the angular position of the pellet to center the laser spot on the pellet. This can be best seen through the back window. You can move the control unit closer to this window during this part of the alignment.
  4. Adjust the position of the substrate to center it in the plume. Make sure it is close enough that the whole substrate is covered by the plume. It may be possible to improve the alignment of the plume on the sample by adjusting the position in inches of the pellet, but this generally shouldn’t be necessary.
  5. Stop firing the laser restore the desired laser rep rate.

 

 

VII. Ablate the film onto the substrate

  1. Select scan pellet move sample on the computer. This will allow you to make last minute adjustments to the substrate position if needed while scanning.
  2. With the shutter open, start firing the laser at integral number of minutes and ablate for the desired time. There is about a 4 sec delay between pressing execute (after pressing run) and the actual first pulse of the laser.
  3. Check the quality of the plume and the location of the beam spot on the target during pellet scanning. Note any anomalies.
  4. Cease firing the laser when the desired time has elapsed and close the shutter.
  5. Repeat VI and VII for the second substrate and pellet if present.

 

 

VIII. Anneal the newly ablated films

  1. I generally anneal at the same temperature as I just ablated at, although I may experiment with annealing temperatures in the future.
  2. Shut off the turbo pump, then the mech. pump. (LEAVE THE COOLING WATER ON.) Shut off the oxygen channel on the flow controller. Open the oxygen bypass valve and fill the chamber to of order 1 atm.
  3. Leave the heater on to anneal the films for the desired time, typically 1 hr. Be sure to continue to record the oxygen pressure and thermocouple temperature on the data sheet graph.
  4. Turn off the control unit and main laser power.
  5. Shut of the heater (by pushing stop), but leave the cooling water on. Allow the samples to cool to room temperature and remove when desired.

 

 

IX. Shut down equipment

  1. When you leave, the heater, the turbo and black roughing pumps and the laser should be off and the computer screen should be dimmed. The temperature controller can be left on, but the MAN indicator should be flashing to indicate that the heater is off.
  2. Once the samples are at room temperature, the cooling water should be turned off.