Optical traps, which use light to hold and move small objects in liquid, are a promising non-contact method for assembling electronic and optical devices. However, when using these traps for manufacturing applications, the liquid must be removed, a process that tends to displace any pattern or structure that has been formed using the optical trap.
In the Optical Society (OSA) journal Optics Express, researchers in Steven Neale's Micromanipulation Research Group at the University of Glasgow detail their method for using an advanced optical trapping approach known as optoelectronic tweezers to assemble electrical contacts.
Thanks to an innovative freeze-drying method developed by Shuailong Zhang, a member of Neale's research group, the liquid could be removed without disturbing the assembled components.
"The forces formed by these optoelectronic tweezers have been compared to Star-Trek like tractor beams that can move objects through a medium with nothing touching them," said Neale. "This conjures up images of assembly lines with no robotic arms. Instead, discrete components assemble themselves almost magically as they are guided by the patterns of light."
The researchers demonstrated the technique by assembling a pattern of tiny solder beads with an optoelectronic trap, removing the liquid, and then heating the pattern to fuse the beads together, forming electrical connections.
Improving Electronics Manufacturing
The new technique could offer an alternative way to make the circuit boards that connect the components found in most of today's electronics.
"The optoelectronic tweezers and freeze-drying technique can be used to not only assemble solder beads, but also to assemble a broad range of objects such as semiconductor nanowires, carbon nanotubes, microlasers and microLEDs," said Zhang. "Eventually, we want to use this tool to assemble electronic components such as capacitors and resistors as well as photonic devices, such as lasers and LEDs, together in a device or system."
The researchers used optoelectronic tweezers because this optical manipulation approach can form thousands of traps at once, offering the potential of massively parallel assembly. The tweezers are formed using a layer of silicon that changes its electrical conductivity when exposed to light. In the areas exposed to points of light, a non-uniform electric field forms that interacts with particles or beads in a liquid layer on top of the silicon, allowing the particles to be precisely moved by moving the point of light. Creating patterns of light points allows multiple particles to be moved simultaneously.
"Using our method, we can move solder beads measuring from the nanometer range up to about 150 microns," said Zhang. "We have been able to move objects that are over 150 microns, but it's more challenging because as the size of the object increases, the frictional force also increases."
After using the optoelectronic tweezers to assemble a pattern of 40-micron-diameter, commercially available solder beads, the researchers froze the liquid in the optoelectronic tweezer device and then reduced the surrounding pressure to allow the frozen liquid to turn from a solid directly into a gas.
In addition to assembling the solder beads into different lines, the researchers also demonstrated parallel assembly of several beads and used the beads to form electrical connections. The solder beads exhibit a strong dielectric force, which enables them to be moved accurately and fast, allowing efficient assembly of structures.
The researchers are now working to turn their laboratory-based system into one that would combine the optoelectronic tweezer and freeze-drying process in a single unit. They are also developing a software interface to control the generation of a light pattern based on the number of particles that needed to be trapped.
"We are now using a computer to generate the light pattern to move the beads, but we are working on an app that would allow a tablet or smart phone to be used instead," said Zhang. "This could allow someone to sit away from the system and use their finger to control the movements of the particles, for example."
In a related engineering project, the first particle was recently quantum teleported to space.
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