FEI DB235 in-situ lift out TEM sample preparation procedure
Nicholas G. Rudawski
ngr@ufl.edu
(805) 252 4916
1.
DISCLAIMER: this procedure assumes the user has experience in basic FIB
operation. In particular, users should have substantial experience preparing exsitu lift outs (EXLOs) before attempting in-situ lift-outs (INLOs), since nearly all of
what is involved in the preparation of EXLOs will transfer to the preparation of
INLOs. To that end, this assumption of basic experience means this procedure
omits many specific details that were present in the EXLO procedure (i.e.,
preliminary sample preparation, how to vent the FIB, load a sample, set eucentric
height, link the beams, navigating through xP, etc.) for the sake of clarity.
2.
Preliminary considerations
2.1.
3.
Special grids are needed to perform INLOs. The most popular are made
by Ted Pella and are referred to as “Omniprobe” grids and are referred to
throughout this procedure. The grids are made of Cu or Mo and have 3,
4, or 5 “fingers” with recessed edges to provide points of attachment for
samples.
Sample/grid loading
3.1.
There is a special holder needed for making INLO samples; this holder
holds both Omniprobe grids. Towards the front of the holder, there are
areas to place C tape to affix specimens (unless you are using a multistub holder). On each side of the holder, there are two slots where an
Omniprobe grid can be loaded. Typically, only one grid is loaded at a time
and that is assumed here. Slightly loosen the knob on the back and
separate the holder to open up the slots.
3.2.
Omniprobe grids come in several different designs. However, all have the
word “Omniprobe” punched out near the bottom of the grid: (Mo, 3 post
1 grid shown here). As viewed here, this is the FRONT side of the grid (side
with the recessed edge). When loading a grid into a slot on the holder,
make sure the front side of the grid faces towards the front side of the
holder; you can use vacuum tweezers (preferred) or a very sharp pair of
regular tweezers to load a grid. Once the grid has been loaded into the
slot, retighten the knob on the back of the holder. Be sure to view the
holder edge on from the front to make sure the grid is level in the slots
(i.e., one side of the grid is not higher than the other). If a grid needs to be
leveled, slightly loosen the knob on the back of the holder and use a pair
of tweezers to level the grid; then retighten the knob.
4.
Holder loading
4.1.
5.
The holder should be loaded so the front of the holder is facing away from
you when the door to the FIB is opened. This correctly orients the grid
relative to the Omniprobe to make attaching the sample to the grid as
easy as possible. Then evacuate the FIB.
Finding an area of interest
5.1.
Turn on the Pt heating, make sure the magnifications are coupled, and
zero the beam shift; when the vacuum is acceptable, turn on the beams.
5.2.
Orient the sample as needed and find an area of interest.
5.3.
Set the area at eucentric height (this is critical; the Omniprobe will
crash if it is inserted with the specimen not at eucentric height).
2 5.4.
Link the beams (set the I-beam scan rotation to 180°), and fine-tune the
scan rotation in the I-beam image (if needed).
5.5.
Make sure the stage is tilted to 52° before proceeding with the rest of the
preparation process.
6.
Deposit a Pt layer using the solid box pattern using a “300 pA” I-beam current;
retract the Pt needle when finished.
7.
Mill deep trenches using the regular cross-section pattern on the top and bottom
sides of the Pt layer using the “5000 pA” I-beam as described in the EXLO
procedure.
8.
Perform rough thinning cuts using the regular cross-section pattern from the top
and bottom sides of the Pt layer using the “1000 pA” I-beam until the sample is 0.5
– 1.0 µm in thickness as described in the EXLO procedure.
3 9.
Undercutting
9.1.
Tilt the stage to T = 7° degrees and delete any present patterns.
9.2.
Draw three solid box patterns. Give one box X and Y dimension of 0.7
and 2.0 µm (left), another with X and Y dimensions of 12.0 and 0.7 µm,
and the other with X and Y dimensions of 0.7 and 4.0 µm.
9.3.
Position the boxes as indicated in the image below and start the patterns.
The specimen will remain attached to bulk in one spot on the left.
10.
Inserting the Omniprobe
10.1.
Start live E-beam imaging.
10.2.
Tilt the stage back to 0°.
10.3.
Reduce the magnification to the lowest possible setting.
4 10.4.
11.
Insert the Omniprobe by entering the
menu and selecting “In” next to
“Omniprobe” under the “Gas Injection” section (the “In” button will turn
yellow and you should be able to hear it actuate and visually observe it
entering the chamber); the probe tip will come in from the upper left corner
of the E-beam image (if the scan rotation for the E-beam = 0°).
Description of Omniprobe controls
11.1.
In the “AutoProbe” program under “Probe Controls” section, either “XY” or
“Z” modes can be selected (the mode can be switched by pressing the
middle icon; i.e., pressing “XY” will change the mode to “Z”). Make sure
“STAGE FOR” and “VEL” are selected or the Omniprobe will not move
correctly. Holding down an arrow of one of the controls moves the
Omniprobe in the specified direction at the indicated velocity. In general,
the closer the Omniprobe is to the specimen, the slower the velocity
needed (to prevent crashing and to enable fine positioning).
11.2.
When viewed in the E-beam mode (with E-beam scan rotation close to
0°), the “XY” control moves the probe left to right (X) and up and down (Y)
in the image; the “Z” control moves the probe along the out of plane
direction (Z).
5 11.3.
12.
When viewed in I-beam mode (with I-beam scan rotation close to 180°),
the “Z” control moves the probe up and down within the image.
Positioning the Omniprobe
12.1.
Enter live E-beam imaging
12.2.
In the AutoProbe software under the “Location Storage” section, select
“Eucentric High” and then press “GOTO”; the probe tip will be moved to a
position directly above the specimen.
12.3.
Increase the magnification to ~1 k× and enter live I-beam imaging. Use
the “Z” control to bring the Omniprobe down towards the sample until it is
a few tens of µm’s from the surface (you will see it enter the image from
the upper right side if the I-beam scan rotation is ~180°).
6 13.
12.4.
Continue to approach the Omniprobe towards the sample. Periodically
switch back to E-beam imaging to make sure the Omniprobe is still
roughly centered over the specimen and adjust the “XY” control, if needed.
12.5.
When the Omniprobe is a few tens of µms from the surface, insert the Pt
needle (this will produce a shift in the E-beam image), and increase the
magnification to ~5 k×.
12.6.
For extraction, the Omniprobe should be centered on the left side of the
sample (as viewed in the E-beam) and clearly touching the specimen
surface (as viewed in the I-beam). Typically, it is necessary to use
velocities ~1.0 µm/s to correctly position the Omniprobe for this.
Attachment and extraction
13.1.
Draw a solid box with X, Y, and Z dimensions 1.0, 1.0, and 0.3 µm.
13.2.
Position it over the point of attachment of the Omniprobe, and deposit Pt
(“pt_tem.mtr” material file) to weld the sample to the Omniprobe (use the
current I-beam setting of “50 pA”).
13.3.
Change the material file to “si.mtr” and select a “300 pA” I-beam current.
13.4.
Change the X,Y,and Z dimensions of the pattern to 0.7, 3.0, and 2.0 µm,
position the pattern over the attached edge of specimen, and start the
pattern. Take frequent E-beam images to observe the progress of the
release cut and stop the pattern when the release cut has completed.
7 13.5.
Erase the pattern, switch the I-beam current to “50 pA”, and enter live Ibeam imaging.
13.6.
Slowly turn the “Z” knob on the chamber door counterclockwise to lower
the stage and pull the bulk sample away from FIB specimen attached to
the Omniprobe (the two should separate easily if the specimen is free and
well-attached to the probe!).
13.7.
Once the specimen is separated from the bulk sample by ~10 µm, use the
“Z” Omniprobe control to raise the specimen up further,
13.8.
When the sample is safely above the surface, retract the Pt needle. DO
NOT retract the Pt needle while the sample is still in the trench and
welded to the Omniprobe; the weld may rupture causing sample
loss.
13.9.
Enter live E-beam imaging and decrease the magnification to the lowest
possible setting.
8 13.10. In the AutoProbe software under the “Location Storage” section, select
“Park” and then press “GOTO” to send the Omniprobe to the “Park”
position (this will take a few seconds and will be completed when the
“GOTO” icon stops reading “MOVING” and reads “GOTO” again).
13.11. When the Omniprobe has reached “Park”, retract it from the chamber by
pressing “Out” next to “Omniprobe” under the “Gas Injection” section of
the
14.
menu.
Attaching the sample to the grid
14.1.
Enter live E-beam imaging.
14.2.
Move the stage over to the grid; if necessary, rotate the grid until it is
aligned with the long axis of the specimen (this makes sample attachment
easier).
14.3.
Center the middle of one of the grid fingers in the image (here, a finger
with a “V” in the middle is used)
14.4.
Set the middle of the finger to eucentric height (again, this is critical; the
Omniprobe will crash if it is inserted with the finger not at eucentric
height).
14.5.
Set the I-beam scan rotation to 180° and link the beams.
14.6.
Reduce the magnification to the lowest possible setting.
14.7.
Insert the Omniprobe by entering the
menu and selecting “In” next to
“Omniprobe” under the “Gas Injection” section.
9 14.8.
in the AutoProbe software under the “Location Storage” section, select
“Eucentric High” and then press “GOTO”; the probe tip will be moved to a
position directly above the finger.
14.9.
Switch to live I-beam mode and bring the Omniprobe down until it is visible
above the grid.
14.10. Swtich back to E-beam mode and center the specimen over the
depression in the recessed edge of the grid.
14.11. When the sample is a few tens of µm’s away from the grid, insert the Pt
needle and increase the magnification to ~5 k×.
14.12. Make sure the specimen is centered over the depression in the recessed
edge of the grid and lower the specimen until the bottom corners are in
light contact with the grid. NOTE: if the sample is not visible in both
images, use the beam shift knobs to adjust one (or both) of the images to
correct this; DO NOT move the stage at this point as this will likely
cause the Omniprobe to crash and the sample to be lost.
10 14.13. Draw a solid box pattern with X and Y dimensions of 1.5 µm and Z
dimension of 0.50 µm and deposit Pt (“pt_tem.mtr” material file) over the
bottom two corners of the specimen touching the grid (sequentially).
15.
Releasing the Omniprobe from the specimen
15.1.
Erase the pattern used to weld the sample to the grid and change the
material file to “si.mtr”.
15.2.
Increase the I-beam current to “300 pA”. Draw a line or solid box pattern
with Z dimension of 10 µm and position over the edge of the probe tip (see
image below) and start the pattern.
11 15.3.
Take periodic single E-beam images during the release cut to check if the
Omniprobe has been freed from the sample; stop the pattern immediately
once the Omniprobe has been freed.
15.4.
Reduce the I-beam current to “50 pA” and enter live I-beam imaging,
slowly move the Omniprobe in the positive Z direction until it is a few µm
away from the sample, then retract the Pt needle.
15.5.
Once the Pt needle has been retracted, put the Omniprobe into the “Park”
position.
15.6.
Once the Omniprobe has reached “Park”, retract it from the chamber by
pressing “Out” next to “Omniprobe” under the “Gas Injection” section of
the
15.7.
menu.
The sample is now ready for finer milling to achieve electron transparency
(not described here). If there are no other samples to be prepared, this
may be done immediately. However, if there are other INLO samples to
be prepared, it is best wait until all samples have been lifted out and
attached to the grid before thinning any sample to the point of electron
transparency; this way, while a specimen is being attached to the grid, any
inadvertent ion beam exposure to samples already attached to the grid
occurs while the samples are still thick, and this damage will be easily
removed during thinning to electron transparency. Alternatively, the
session may be ended without further thinning the sample(s) and the grid
reloaded and thinning to electron transparency performed during a later
session.
12 16.
Finishing the session and storing specimens
16.1.
If no further work is to be performed, finish the session as normal (turn
beams off, vent, initialize stage, log of the TUMI system, etc.), remove the
holder from the stage (be careful not to touch the grid or it may get
mangled and/or samples might be torn off), and pump the FIB back down.
16.2.
Remove the grid from the holder by very slightly loosening the screw on
the back of the holder and using a sharp pair of tweezers to grab the edge
of the grid (away from the fingers where sample are attached) and extract
the grid from the holder.
16.3.
Place the grid front side UP for storage (best to prevent the samples from
touching any surfaces and/or possibly being torn off). If possible, use a
special box designed for storing Omniprobe grids.
13