Improvements to computer chips are usually associated with transistors, but the components that deliver power to them are no less important. For a chip to work stably, the voltage must be right and fluctuations must be kept under control. Shrinking such devices is a separate task, and that is exactly what researchers from the University of Florida, together with colleagues, have taken on.
They fabricated tiny magnetic devices - strip microinductors. An inductor is a component that temporarily stores energy in a magnetic field and helps smooth out changes in current. The work used a composite of iron oxide and nickel nanoparticles: the particles are embedded in a metal matrix. Measurements showed that the inductance of such devices is more than twice that of comparable samples without a magnetic core.
The main question of the research is how to integrate useful magnetic materials into the small structures used in semiconductor manufacturing. The work was published in the Journal of Micromechanics and Microengineering. The authors include Brenden M. Mears, Noah Morgan, Jennifer S. Andrew and David P. Arnold.
Inductors are hard to shrink: as dimensions decrease, inductance usually falls. One way to compensate is to add magnetic material around the conductor. Another is to raise the switching frequency of the voltage regulator, but then the material must respond quickly to changes and not lose too much energy.
To create the composite, electroinfiltration was used: iron oxide nanoparticles were surrounded by nickel, which was electrochemically deposited into the gaps between them. The resulting material was placed in a strip microinductor, with an insulated copper strip positioned between the bottom and top magnetic layers. To make the top layer lie evenly, a photoresist softened by heating - a light-sensitive material - was used, and this increased the copper thickness from about 2 to 6 micrometers, reducing the winding resistance.
The devices showed an inductance of about 1.6 nanohenries. A nanohenry is one billionth of a henry, a unit of inductance; for high-frequency circuits this is a significant value. In addition, the quality factor (Q) was assessed - an indicator of the balance between inductive behavior and electrical losses at a given frequency. The optimized device reached a peak quality factor of 5.3 at 150 megahertz, which is 58% higher than the original process with a peak of 3.35. This is a measurement of an individual component, not the efficiency of a finished voltage regulator.
The authors emphasize that higher inductance by itself does not guarantee a better power converter. Performance is affected by resistance, magnetic losses, current rating and temperature. In an additional test at direct current, the inductance of one of the co-fabricated inductors remained approximately stable up to 330 milliamperes at 1 megahertz - in this range no noticeable magnetic saturation was detected. However, this does not establish the current rating of the optimized device at 150 megahertz.
Questions of heating, reliability and operation as part of complete converters lie ahead. The device yield also needs improvement: of 97 tested samples, 64 worked. According to the authors, the results create a basis for integration at the component level, rather than demonstrating a reduction in the power consumption of a finished processor.