Researchers from MIT have come up with a miniature “electroadhesive” stamp pick-and-place solution capable of handling objects as small as 20 nanometers wide. To put it to scale, that’s about 5,000 times finer than human hair.
As components become smaller and smaller, robotic grippers are also approaching a limit in picking up and placing them down in precise configurations.
“Electronics manufacturing requires handling and assembling small components in a size similar to or smaller than grains of flour,” says Sanha Kim, a former MIT postdoc and research scientist who worked on the research. “So a special pick-and-place solution is needed, rather than simply miniaturizing [existing] robotic grippers and vacuum systems.”
This stamp is made from ceramic-coated carbon nanotubes that are arranged like bristles on a tiny brush. A small voltage activates the stamp, creating a temporary charge in the carbon nanotubes. The electric charge formed attracts a minute particle, enabling an object to “stick” to the stamp. When the voltage is switched off, the “stickiness” disappears, releasing the object.
“Simply by controlling voltage, you can switch the surface from basically having zero adhesion to pulling on something so strongly, on a per unit area basis, that it can act somewhat like a gecko’s foot,” says John Hart, a mechanical engineering associate professor at MIT.
According to Hart, they were aware that electroadhesion has been used in industrial settings for picking and placing large objects—fabrics, textiles, and even silicon wafers. However, the process has never been attempted at the microscopic level. This is because it would require a completely new material design to be capable of controlling electroadhesion at a smaller scale.
Existing mechanical grippers are not capable of picking up objects smaller than 50 to 100 microns. This is because surface forces tend to win over gravity at smaller scales. An example of this phenomenon is when tiny flour particles stick to a spoon no matter how hard you try to shake them off.
“The dominance of surface forces over gravity forces becomes a problem when trying to precisely place smaller things—which is the foundational process by which electronics are assembled into integrated systems,” Hart says.
Hart and his team have previously worked with carbon nanotubes (CNTs)—atoms of carbon linked in a lattice pattern and rolled into microscopic tubes—which have been widely studied as dry adhesives.
“Previous work on CNT-based dry adhesives focused on maximizing the contact area of the nanotubes to essentially create a dry Scotch tape,” Hart says. “We took the opposite approach, and said, ‘let’s design a nanotube surface to minimize the contact area, but use electrostatics to turn on adhesion when we need it.’”
The team discovered that by coating CNTs with a thin dielectric material such as aluminum oxide, the ceramic layer becomes polarized when voltage is applied to the nanotubes. This results in the nanotube stamp being able to pick up an object, like tiny electrostatic fingers. Similarly, when the researchers switched the voltage off, the ceramic layer depolarized and the stamp detached.
Various formulations for the stamp’s design were explored. The density of the carbon nanotubes grown on the stamp and the thickness of the ceramic layer coating were altered until arriving at the most optimal conditions. According to the team, they discovered that the thinner the ceramic layer and the more sparsely spaced the carbon nanotubes, the greater the stamp’s on/off ratio. This means it was more “sticky” when the voltage was on than when it was turned off.
To test the stamp, it was used to pick up and place down films of nanowires 5,000 times thinner than human hair. Additionally, it was also tested to pick and place intricate patterns of polymer, metal microparticles, and micro-LEDs.
Hart expressed how the process could eventually be scaled up to manufacture circuit boards and systems of miniature electronic chips, and even displays with microscale LED pixels.
“With ever-advancing capabilities of semiconductor devices, an important need and opportunity is to integrate smaller and more diverse components, such as microprocessors, sensors, and optical devices,” Hart says. “Often, these are necessarily made separately but must be integrated together to create next-generation electronic systems. Our technology possibly bridges the gap necessary for scalable, cost-effective assembly of these systems.”
The results from the research are published in the Science Advances journal.
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