Optical Tweezer Assembles Electronic Components

Researchers used optoelectronic tweezers to assemble a line of solder beads. By removing the liquid with a freeze-drying approach, the assembled beads stay fixed after the liquid is removed. (Source: S. Zhang)

An inter­national team of researchers has developed a new light-based mani­pulation method that could one day be used to mass produce electronic components for smart­phones, computers and other devices. A cheaper and faster way to produce these compo­nents could make it less expensive to connect everyday objects to the internet, advan­cing the concept known as the Internet of Things. The micromanipulation technique might also be used to create a safer and faster-charging replace­ment for mobile device batteries.

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 an optical trap. Researchers in Steven Neale’s Micro­manipu­lation Research Group, University of Glasgow, Scotland, report about 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. They used the solder beads to demonstrate that in the future, these micro­particles could be assembled and fused to create elec­trical connec­tions.

“Opto­electronic tweezers are cost-effec­tive and allow parallel micro­manipu­lation of particles,” said Zhang, who is now at the Univer­sity of Toronto in Canada. “In principle, we can move 10,000 beads at the same time. Combining this with our freeze-drying approach creates a very inexpen­sive platform that is suitable for use in mass produc­tion.” The new technique could offer an alter­native way to make the circuit boards that connect the components found in most of today’s elec­tronics. These types of devices are currently made using auto­mated machines that pick up tiny parts, place them onto the circuit board and solder them into place. This process requires an expensive motorized stage to position the board and a costly high-precision robotic arm to pick up and place the tiny parts onto the device. The cost of these micro­manipula­tion systems continues to rise as the shrinking size of elect­ronics increases precision require­ments.

“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 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. This freeze-drying approach allowed the assembled solder beads to remain fixed in place after the liquid was removed. The researchers say it can be used to remove liquid used with any type of optical trap, or even traps formed with acoustic waves.

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 means they can be moved accurately and fast, allowing very 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.” Neale recently received funding to continue this line of research by using the new optical micro­manipula­tion approach to create high energy density capa­citors to replace batteries in mobile devices. (Source: OSA)

Reference: S. Zhang et al.: Manufacturing with light – micro-assembly of opto-electronic microstructures, Opt. Exp. 25, 28838 (2017); DOI: 10.1364/OE.25.028838

Link: School of Engineering, Univ. of Glasgow, Glasgow, Scotland, UK

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