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By Kimberly Tuck, Zyvex Instruments
Download a printable copy here.
Introduction
The S100 platform of robotics tools can be used for electrical
characterization, failure analysis, materials evaluation and
physical property measurements. The ability to address such
a wide array of applications is enabled by a suite of end-effectors
which can be employed to probe, grasp, bias, oscillate, and
manipulate objects and materials on the micro- and nanoscale.
Such capabilities at this level, which until recently were
commercially unavailable, are standard features of the S100
Nanomanipulator System.
One of the most useful manipulation end-effector tools used
with the S100 is a Microelectromechanical System (MEMS-based)
microgripper. This tool allows small delicate objects to be
maneuvered with nanometer-scale precision and accuracy. Microgrippers
have uses in a number of applications including Transmission
Electron Microscope (TEM) sample preparation, microassembly,
and materials analysis where biological or inorganic materials
must be held, stretched, or moved. The size of the sample
being manipulated determines which gripper is used.
Background
Zyvex microgrippers function on the basis of electrothermal
actuation, a technique which enables large deflections and
high gripping forces. The gripping motion is achieved from
thermal expansion and motion amplification which has been
optimized to achieve appropriate deflection at the tips with
sufficient force. These grippers have been designed for user-controlled
movement with a given power input.
The microgripper is packaged onto a heat dissipating sub-mount
(Figure 1).
Figure 1. Microgripper sub-mount and
dimensions
The microgrippers can be mounted at either 90° or at
180° on the sub-mount (Figure 2).
Figure 2. Microgripper with horizontal
(left) or perpendicular (right) sub-mount
The orientation of the part being manipulated and grasped
decides the orientation of the microgrippers. The sub-mount
is placed into an end-effector interface (Figure 3)
which connects the end-effector assembly to the S100 positioner.
Figure 3. Microgripper end-effector
interface
The interface has been designed to connect the electrical
traces from the sub-mount to a circuit which inserts into
the positioner via a 5-pin connector plug. The holder is also
designed to shield the ceramic sub-mount to avoid charging
problems inside the Scanning Electron Microscope (SEM). The
combination of the gripper, the sub-mount, and the interface
comprise the microgripper end-effector for the S100. Figure
4 shows the S100 head unit with four X, Y, Z positioners
and a mounted microgripper end-effector.
Figure 4. S100 head assembly with microgripper
end-effectors installed
Zyvex offers an array of gripper designs tailored to a multitude
of applications. Factors to consider when choosing a gripper
include grasping force, gripper opening, and pincher dimensions.
For instance, a different gripper would be used for TEM sample
preparation than for microcomponent assembly.
Since the grippers can close completely, there is practically
no lower limit on gripped feature size. Carbon nanotubes with
a diameter of 10 nanometers or less can be manipulated. Our
largest gripper can handle components approximately 500 microns
in size.
There are different styles and sizes of MEMS grippers that
have been designed to accommodate different tasks. The voltage
necessary to close the grippers varies depending on the design
and the fabrication process.
Applications
Figure 5 shows a 50 µm thick SCS grippers with a minimum
gap opening of 36 µm when unpowered, opening up to a
maximum opening of
80 µm.
Figure 5. 50 micron thick SCS microgripper
This gripper design is used for pick and place of other MEMS
components. Figure 6 shows these grippers
being used to grasp and place an iron beam onto an assembly.
Figure 6. Pick and place using microgrippers
This electroplated nickel iron beam is grasped and placed
onto a MEMS translating stage. As the stage is translated,
the nickel iron beam moves within a magnetic field to change
inductance properties. These grippers have also been used
to create 3D structures, by picking and placing MEMS connectors
which are then inserted into specifically designed MEMS sockets
(Figure 7).
Figure 7. MEMS connectors inserted
in sockets
Figure 8 displays a pair of 2 µm thick
grippers that are actuated using the bimorph principle. This
gripper is manipulating a polysilicon block, and uses two
bimorph actuators oriented so that the tips move toward each
other when current passes through the device.
Figure 8. Bimorph-type grippers manipulating
a MEMS component
A typical MEMS bimorph is shown in Figure 9.
Figure 9. An electrothermal bimorph
MEMS device
Current passes through the device from anchor to anchor.
There is higher current density in the thinner arm (hot arm),
and higher resistance, which causes joule heating. The joule
heating causes the hot arm to expand more than the cold arm.
The arms are joined at the end, which constrains the tip of
the actuator making it move laterally in an arc motion toward
the wider arm.
The grippers shown in Figure 10 are different from those
in Figure 8.
Figure 10. Scaled MEMS microgrippers
5 µm thick (left) and 50 µm thick microgripper
(right)
Although the gripping motion is achieved from thermal expansion,
the motion is amplified and optimized for maximum deflection
of the tips using a bent beam configuration (as shown). The
beams thermally expand causing a downward force which then
causes the gripper tips to squeeze together. This design has
been used to make grippers with minimum feature size of 500
nm (scaled MEMS) as shown in Figure 10 - left. These scaled
MEMS grippers are 5 µm thick and are capable of closing
completely. The same grippers shown in Figure 10 -
left have been used for TEM sample lift-out, and
have picked up smaller MEMS components. They have also been
used to pick up nanotubes. These grippers have also been designed
in another process where the grippers are 50 µm thick
and the minimum feature size is 5 µm. The grippers in
Figure 10 - right have been used to perform
various pick and place experiments assembling connectors and
creating 3D structures.
Carbon nanotubes are notoriously difficult material to handle
individually. The tubes clump and bind together in bundles,
making extraction of individual strands difficult. The task
is made much simpler using an S100 and a microgripper end-effector.
Figure 11 shows a single carbon nanotube
strand, about 10 nm in diameter, being removed from a bundle.
Figure 11. Gripper grasping a single
carbon nanotube
Figures 12 shows one of the most exciting
recent applications for microgrippers. The top two images
depict the approach of the gripper into a FIB-cut coupon trench.
The bottom two images show the acquisition and removal of
the FIB-cut coupon. With five degrees of freedom available
with the S100 Nanomanipulator, coupled with sub-5 nm movement
resolution, the FIB-cut coupon can be lifted, moved, and placed
very accurately on a TEM grid.
Figure 12. MEMS gripper picking up
FIB-cut coupon
Conclusion
Until recently, accurately acquiring and moving components
and materials with dimensions less than 1 mm was a formidable
task. Tools did not exist which could mechanically grasp elements
with feature sizes that small. The engineer/technician was
forced to use push-pull motions using needles, or in many
cases, just admit defeat. The difficult challenges of manipulating
micro- and nanoscale objects are overcome using the precision-guided
S100 with microgripper end-effectors.
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