Electric fields assemble devices

By Eric Smalley, Technology Research News

The easiest way to assemble a device is to provide an environment that causes it to self-assemble. As components continue to shrink, this could become the only practical means of assembly.

In contrast, today's electronic devices are generally put together using tiny equivalents of assembly line robots that pick up and place components on chips. This approach can be scaled down only so far because at a certain point natural forces cause components to stick to assembly mechanisms.

Researchers from the National Microelectronics Research Centre (NMRC) in Ireland have sidestepped the problem by using electric fields to direct the assembly process. They used the technique to cause arrays of gallium arsenide light-emitting diodes to assemble onto silicon chips.

The method, dubbed Field Controllable Assembly, is compatible with contemporary optoelectronics manufacturing methods, and could lead to low-cost, rapid assembly of optical and electronic devices, said Alan O'Riordan, a research scientist at the National Microelectronics Research Centre.

The method takes advantage of the electric charge that most objects carry. "The response of these charges to an appropriately configured electric field may be exploited to direct the transport and ultimately the self-assembly of the devices," O'Riordan said.

The researchers' self-assembly device contains an array of electrodes on a silicon surface that allows them to put electric fields of specific configurations on the surface of the chip. The fields can be configured to attract electric charges at a particular spot and repel it everywhere else. The right configuration causes components to rapidly move from one spot to another. Once the components are in place, the transport fluid is evaporated and the components fused to the surface by melting and cooling the chip's tin-gold contacts.

In theory, the technique could move components over distances of up to several millimeters, according to O'Riordan.

The researchers' method is a high-throughput, non-contact, pick-and-place technique that allows for programmable manipulation of individual components on silicon surfaces like computer chip wafers, said O'Riordan.

A key factor of the researchers' self-assembly technique is that, unlike many natural self-assembly processes, the objects involved are not required to recognize the parts they must connect to.

The researchers used the method to assemble devices from 50- to 80-micron diameter light-emitting diodes, and to transport 1-micron latex beads. A micron is one thousandth of a millimeter; the width of a human hair is about 75 microns.

The method is scalable down to the nano level, and could eventually be used to assemble nanoelectronic, nanophotonic, and nanoscale biotechnology devices, said O'Riordan. A nanometer is one millionth of a millimeter.

The technique can also be used to assemble many components in parallel. As long as the spots are about the same diameter as the components, the method assures that each spot attracts a single component, according to O'Riordan. For example, the researchers randomly deposited a set of light emitting diodes in a liquid on a chip surface and configured the electric fields on the chip to attract the diodes to an array of spots.

The method is particularly applicable to microelectrical mechanical systems (MEMS), which incorporate moving parts into computer chips, and optoelectronics, which convert light signals to electrical signals and vice versa. The cost reduction the method would bring about would be particularly useful in these manufacturing sectors, said O'Riordan.

The method could be used practically in two to five years, said O'Riordan.

O'Riordan's research colleagues were Paul Delaney and Gareth Redmond. The work appeared in the May, 2004 issue of Nano Letters. The research was funded by the the European Union and Enterprise Ireland.

Timeline:   2-5 years
Funding:   Government
TRN Categories:  Nanotechnology; Integrated Circuits
Story Type:   News
Related Elements:  Technical paper, "Field Configured Assembly: Programmed Manipulation and Self-assembly at the Mesoscale," Nano Letters, May, 2004; video of Field Configurable Assembly of light-emitting diodes, www.nmrc.ie/research/nanotechnology-group/research_topics/Self_Assembly/Self_Assembly_FCA.HTML


July 28/August 4, 2004

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