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By Kimberly Tuck, Zyvex Instruments
Download a printable copy here.
The Zyvex Nanoprober System, coupled with a focused
ion beam (FIB) tool, is a complete solution for TEM sample
preparation. The Zyvex System has the ability to perform the
following tasks in a dual beam FIB (dbFIB) environment (see
Figure 1).
• Probe bulk samples
• Take electrical measurements
• Manipulate samples
• Perform immediate in-situ transmission electron
microscope (TEM) sample lift-out
Figure 1: (a) Zyvex Nanoprober probing (b)
Zyvex Nanoprober cutting samples
(c) Zyvex Nanoprober lift-out
The ability to prepare site-specific specimens (that can
be removed from the bulk of a sample) provides more insight
into defect analysis, process monitoring and control, and
materials characterization of increasingly smaller areas.
This is an extremely important application.
The Zyvex Nanoprober (Figure 2) is capable
of performing the standard in-situ lift-out method
using a tungsten probe along with FIB micro-machining and
welding. Zyvex has developed a new approach to TEM sample
lift-out by using NanoEffector™ microgrippers to grasp
the FIB-cut samples. This unique approach allows samples to
be removed and placed onto a TEM grid inside either a scanning
electron microscope (SEM) or an FIB system using, in-situ,
the Zyvex Nanoprober.
Figure 2: Zyvex Nanoprober System
The new Zyvex NanoEffector microgripper-based method is favorable
over traditional methods since samples can be removed and
manipulated without being contaminated by FIB deposition.
Zyvex has developed an automated assembly technology for microsystems
which is scaleable to nanosystems manipulation and assembly.
Much of this research and development has been done using
electrothermal micro-electromechanicial systems (MEMS)-based
microgrippers to assemble many three-dimensional structured
microsystems (see Figure 3). MEMS grippers
allow small, delicate objects to be maneuvered with nanometer-scale
precision and accuracy. These grippers have other assembly
applications, including TEM sample preparation.
Figure 3 (a-c) Zyvex technology using microgrippers
Zyvex NanoEffector Microgrippers (see Figure 4)
are used for TEM sample lift-out. The grippers use electrothermal
actuation to achieve a range of motion from zero to five microns
of opening at the tips. An electrical potential is applied
across the two anchor pads, passing current through the bent-beam
structures. Joule heating within the beams results in localized
thermal expansion. This thermal expansion drives flexures
which amplify and create horizontal motion at the gripper
tips.
Figure 4: Power-open electrothermal grippers
In the MEMS fabrication process, there is a minimum spacing
between the tips. Zyvex packages the NanoEffector Microgrippers
onto an aluminum nitride sub-mount.
The sub-mount, with microgrippers attached, is then mounted
on the end of the Zyvex Nanoprober plug. This gives the microgrippers
the ability to move in x, y, z, and theta
to grasp the sample and lift it out. The range of motion of
the Nanoprober is excellent for this task.
The Zyvex System is capable of performing the standard method
of in-situ TEM lift-out using Zyvex NanoEffector
tungsten probes. After locating the area of interest, a sample
is micromachined in the dbFIB system. One of the four probes
on the Nanoprober is then inserted into the area of interest.
The probe is placed on top of the sample and then welded to
the sample using FIB deposition. The tethers are cut to ensure
the sample is entirely free. To lift out the sample and find
the TEM grid, the Zyvex Nanoprober then moves up in z and
over in x and y. The sample is then welded to the TEM grid
for further thinning (if necessary). The steps to this process
are shown in Figure 5.
Figure 5 (a-d): In-situ probe method
for TEM lift-out
Normal welding techniques are not always reliable because
the probe does not weld very securely to the sample using
the platinum deposition. It is easy to break the platinum
deposition if the sample is not completely free at all times
while trying to move the probe. Even vibrations from the gas
injector can cause the platinum to crack.
It should be noted that this method requires some maintenance
to the probe to keep it sharp. The tip must be re-etched every
time it becomes dull from the welding and cutting contamination
process. This method also risks contaminating the sample.
Current methods of probe in-situ lift-out are cumbersome,
time consuming, and may contaminate the sample in the process.
Zyvex believes there is a better solution to this task. By
eliminating the steps of welding and milling the probe to
the sample, all of the limitations above can be surmounted.
The Zyvex lift-out method uses NanoEffector Microgrippers
powered by electrothermal actuators. These state-of-the-art
microgrippers allow the sample to be easily grasped and manipulated
freely — without risking contamination. When Zyvex’s
microgrippers are powered, they open. When the power is turned
off the microgrippers close around the sample in order to
retrieve it from the FIB-cut trench. The gripping motion is
achieved by thermal expansion and motion amplification which
has been optimized to achieve appropriate deflection with
sufficient force at the tips.
Ideally, for efficient TEM lift-out, the samples are picked
and placed in a serial automated fashion onto a carbon-backed
TEM grid. This process would be useful for a very high efficiency
design. If a higher quality of analysis is necessary, and
efficiency is not as much of a concern, the samples could
be welded onto the side of a TEM grid.
With the Zyvex microgripper lift-out process, it is possible
to retrieve FIB-cut samples of varying sizes since the micro-grippers
have a variable opening.
Another benefit of this new process is that the sample need
not be completely cut free to lift it out. Our samples are
undercut and are left with only one tether. The gripping force
causes the tether to controllably break during lift-out.
As shown in Figure 6, the grippers are aligned
over the sample using the Nanoprober. The power is turned
on to open the grippers above the sample.
Figure 6 (a-c): Preparing to lift-out using
MEMS grippers
Then the grippers move down, inserting the gripper tips around
the sample. The power is turned off and the sample is grasped
as the tether is broken. The sample is securely grasped and
is then removed from the trench. Figure 7 shows
the lift-out sequence.
Figure 7 (a-c): Lifting out the sample with
MEMS grippers
The final step is to locate the sample to the TEM grid by
moving in x and y and rotating the stage. Then the sample
is placed into the groove of the grid, and welded in place.
It is also possible to set the sample down by opening the
gripper to release it. You can also perform this process on
a carbon backed grid. Figure 8 shows the
sample placed onto the side of the grid as a demonstration
of the ability to release the samples without stiction. Welding
to the side of the grid is necessary for re-thinning the sample.
Figure 8 (a-c): Placement of sample onto
grid
The Zyvex microgripper lift-out process has many advantages
over the standard probe lift-out method. It avoids many of
the cutting and welding steps which shortens the process—saving
time. Contamination is also less likely to occur since the
tool is not welded to the sample. Also the microgrippers can
be re-used for hundreds to thousands of lift-outs without
requiring maintenance to maintain their performance. The standard
probe lift-out method requires re-sharpening in the FIB after
each sample lift-out. The Zyvex Nanoprober, with better than
5 nm resolution and 4 degrees-of-freedom, makes manipulation
controlled and smooth.
NanoEffector Microgrippers used with the Zyvex Nanoprober
for TEM sample lift-out is a new approach that overcomes some
of the current limitations of probe in-situ lift-out.
It is capable of performing lift-out with two processes —
from four different orientations. The key advantages of the
NanoEffector microgripper lift-out method are: faster lift-out
times, less contamination, reliable process, reusable, no
maintenance, and easy to use. Future developments for this
application will implement automation to minimize user interaction
and increase efficiency.
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