Procedure for Laser Ablation
of YBCO
(Rev 2 10/24/97)
O. General
- 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.
- The yellow safety goggles must be worn and the
warning light by the door must be turned on whenever the
laser is being fired.
- 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.
- Cooling water MUST BE ON and flowing to the
proper locations whenever the sample heater or the turbo
pumps are in use.
- 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)
- Mount substrate on polisher chuck using crystal bond and
hot plate. Allow to cool.
- Select desired bowl and polishing slurry.
- Mount chuck in collar with set screw. Set up
bowl and chuck in polisher.
- Polish until all old material is removed and a
shiny finish is obtained.
- Remove chuck from collar. Reheat chuck to
loosen crystal bond and remove substrate from chuck.
- Clean substrate in acetone, isopropyl, and
methanol
- Clean chuck and collar. Use acetone to remove
crystal bond, DI water to remove water based slurries, or
isopropyl to remove oil based slurries.
- If a water based slurry was used (typically
mastermet), clean the bowl thoroughly with water.
- 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
- 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.
- Remove the targets by unscrewing them with your
left hand while holding the mounting screw in your right
hand, so that it wont 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.
- 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.
- If a target comes off of its copper chuck, it can
be reattached with silver paint in a similar manner to
substrate mounting (described below).
- Blow the dust off the targets with a
Chemtronics can or with nitrogen.
- 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
- 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.
- The shuttle with the samples on it should be
left in the dessicator to dry for at least 4 hours,
preferably overnight.
- 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.
- 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.
- 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.)
- 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 doesnt happen, you should be able
to reset the turbo pump by holding down the reset switch.
- 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
- 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.
- 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 cant get enough max. power, you
will have to refill the laser gases
- 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
- These steps can be interspersed with the steps in IV once
you are familiar with everything.
- 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.
- 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.
- 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.
- 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.)
- 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.
- 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.
- 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
- Set the laser rep rate to 1 Hz. Open the shutter.
- 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.
- 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.
- 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 shouldnt be necessary.
- Stop firing the laser restore the desired laser
rep rate.
VII. Ablate the film onto the substrate
- 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.
- 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.
- Check the quality of the plume and the location
of the beam spot on the target during pellet scanning.
Note any anomalies.
- Cease firing the laser when the desired time
has elapsed and close the shutter.
- Repeat VI and VII for the second substrate and
pellet if present.
VIII. Anneal the newly ablated films
- I generally anneal at the same temperature as I just
ablated at, although I may experiment with annealing
temperatures in the future.
- 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.
- 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.
- Turn off the control unit and main laser power.
- 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
- 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.
- Once the samples are at room temperature, the
cooling water should be turned off.