Aqueous processes spill over at IEDM

Aqueous processes spill over at IEDM
NEW YORK — Aqueous solutions will take the spotlight at the International Electron Devices Meeting in San Francisco next month. A number of presentations at the 46th annual IEDM (Dec. 11-13) will describe how aqueous-based principles are being applied to build sensors and circuits.

Researchers at Silicon Biosystems (Bologna, Italy) will describe a method for stable levitation and independent motion of microorganisms in a liquid and their precise programmable displacement. Miniature bioanalytical instruments such as labs-on-a-chip need faster and cheaper laboratory processes. At the same time, scaling down microelectronics feature sizes makes it possible to build circuits that are comparable in size to microorganisms such as cells and bacteria.

The goal of the researchers is to realize a particle manipulator in standard CMOS technology based on programmable dielectrophoresis (DEP)-force cages, where single particles may be trapped, kept in levitation and dragged above a chip's surface.

Since microorganisms and cells are electrically neutral, they can be controlled using DEP forces with increasing or decreasing field intensity. The forces are used to displace cells in a microchamber formed between two facing glass chips with elongated electrodes. Programming allows the planning and execution of a large number of experiments without changing the IC, the team said, creating a flexible platform that can be used for many applications. A movie showing the displacement of microorganisms will be shown at the group's IEDM paper presentation.

In another experimental foray to control bioanalytical experiments, researchers from the IMEC research consortium in Leuven, Belgium, will describe their first attempt to use an organic-based field-effect transistor as a sensing device for charge detection in aqueous media. In a first step to realize all-plastic, disposable devices in bioanalytical applications, the IMEC team fabricated organic-based FETs able to detect protons.

Silicon-based ion-sensitive FETs have been used for more than 30 years to detect charge in aqueous media, but their response drift has kept them from becoming practical in volume. IMEC researchers circumvented the drift problem by developing disposable, organic-based versions.

A window is etched in the silicon substrate to expose the silicon nitride dielectric to solutions of different pH values. The I-V characteristics measured for different pH values, at the same bias conditions, allowed the researchers to extract a variation of the source-drain current of approximately 30 nA/pH unit. This is a reproducible result and proves the pH sensitivity of the organic transducer, the team said.

Meanwhile, the electrical engineering and computer science departments at the University of California, Berkeley, are pursuing fluidic self-assembly. This nanotechnolgy process enables high-density integration of incompatible technologies such as optical emitters with silicon. Wafers are micromachined into trapezoidal elements, released and processed as slurry — a watery mixture of insoluble elements.

Researchers at Berkeley are able to manipulate and control the hydrophilic/hydrophobic interactions of the surfaces in an aqueous solution by dispersing the elements over a target substrate of matching geometry to the elements. The accuracy of the self-orientation process is limited only by the micromachining accuracy.

In this way, for example, high-performance gallium arsenide and silicon devices each can be grown in their native single-crystal substrates, and then seamlessly integrated. Dense-enough interconnects and small-enough parasitics have been achieved to demonstrate 0.9999 yields for assemblies with thousands of elements. A videotape of an assembly will be shown at the conference presentation.

The ability to take images of both dry and wet fingers is driving research at Ethentica (Lake Forest, Calif.). Researchers there are developing a thin, optical fingerprint-image sensor based on a hydrogenated amorphous-silicon pin photodiode matrix array.

The sensor relies on differential pathways of photons that can be scattered, absorbed or reflected in the presence or absence of skin. Ridges show up as darker areas because of absorption, while valleys are lighter because light gets reflected.

The 315 x 240-pixel sensor uses a switched diode for photocharge readout and a photodiode to detect the reflected photons. Images recorded with a normal finger and a wet finger showed similar excellent results, the team said, and will be presented at IEDM.