NEW PROCESS CREATES 3-D NANOSTRUCTURES WITH MAGNETIC MATERIALS

The following information was released by the National Institute of Standards and Technology:

Materials scientists at the National Institute of Standards and Technology (NIST) have developed a process to build complex, three-dimensional nanoscale structures of magnetic materials such as nickel or nickel-iron alloys using techniques compatible with standard semiconductor manufacturing. The process, described in a recent paper,* could enable whole new classes of sensors and microelectromechanical (MEMS) devices.

The NIST team also demonstrated that key process variables are linked to relatively quick and inexpensive electrochemical measurements, pointing the way to a fast and efficient way to optimize the process for new materials.

The NIST process is a variation of a technique called "Damascene metallization" that often is used to create complicated three-dimensional copper interconnections, the "wiring" that links circuit elements across multiple layers in advanced, large-scale integrated circuits. Named after the ancient art of creating designs with metal-in-metal inlays, the process involves etching complex patterns of horizontal trenches and vertical "vias" in the surface of the wafer and then uses an electroplating process to fill them with copper. The high aspect ratio features may range from tens of nanometers to hundreds of microns in width. Once filled, the surface of the disk is ground and polished down to remove the excess copper, leaving behind the trench and via pattern.

The big trick in Damascene metallization is ensuring that the deposited metal completely fills in the deep, narrow trenches without leaving voids. This can be done by adding a chemical to the electrodeposition solution to prevent the metal from building up too quickly on the sides of the trenches and by careful control of the deposition process, but both the chemistry and the process variables turn out to be significantly different for active ferromagnetic materials than for passive materials like copper. In addition to devising a working combination of electrolytes and additives to do Damascene metallization with nickel and a nickel-iron alloy, the NIST team demonstrated straightforward measurements for identifying and optimizing the feature-filling process thereby providing an efficient path for the creation of quality nanoscale ferromagnet structures.

The new process makes it feasible to create complex three-dimensional MEMS devices such as inductors and actuators that combine magnetic alloys with non-magnetic metallizations such as copper interconnects using existing production systems.

* C.H. Lee, J.E. Bonevich, J.E. Davies and T.P. Moffat. Magnetic materials for three-dimensional Damascene metallization: void-free electrodeposition of Ni and Ni70Fe30 using 2-mercapto-5-benzimidazolesulfonic acid. Journal of The Electrochemical Society, 155 (7) D499-D507 (2008).

Media Contact: Michael Baum, michael.baum@nist.gov, (301) 975-2763

Exposing the Sensitivity of Extreme Ultraviolet Photoresists

NIST researchers exposed a 300 mm silicon wafer with incrementally increasing doses of extreme ultraviolet light (EUV) in 15 areas. After the wafer was developed, the team determined that the seventh exposure was the minimum dose required (E0) to fully remove the resist.

Credit: NIST

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Researchers at the National Institute of Standards and Technology (NIST) have confirmed that the photoresists used in next-generation semiconductor manufacturing processes now under development are twice as sensitive as previously believed. This finding, announced at a workshop last month,* has attracted considerable interest because of its implications for future manufacturing. If the photoresists are twice as sensitive as previously thought, then they are close to having the sensitivity required for high volume manufacturing, but the flip side is that the extreme ultraviolet optical systems in the demonstration tools currently being used are only about half as effective as believed.

Extreme ultraviolet lithography (EUVL) is a process analogous to film photography. A silicon wafer is coated with photoresist and exposed to EUV light that reflects off a patterned "photomask." Where the light strikes the resist it changes the solubility of the coating. When developed, the soluble portions wash away leaving the same pattern exposed on the silicon surface for the processing steps that ultimately create microcircuits.

The drive to make circuits with ever smaller features has pushed manufacturers to use shorter and shorter wavelengths of light. EUVL is the next step in this progression and requires developing both suitable light sources and photoresists that can retain the fine details of the circuit, balancing sensitivity, line edge roughness and spatial resolution. NIST researcher Steve Grantham says that optical lithography light sources in use today emit light with a wavelength of about 193 nanometers, which borders on optical wavelengths. EUVL sources produce light with wavelengths about an order of magnitude smaller, around 13.5 nanometers. Because this light does not travel through anything"including lenses"mirrors have to be used to focus it.

Until recently, EUV photoresist sensitivity was referenced to a measurement technique developed at Sandia National Labs in the 1990s. Late in 2007, scientists at the Advanced Light Source at Lawrence Berkeley National Laboratory in Berkeley, Calif., used a NIST-calibrated photodetector to check the standard. Their detector-based measurements indicated that the resist' sensitivity was about twice that of the resist-based calibration standard.

Following on the intense interest that these results generated when the Berkeley group presented them at a conference in February, the Intel Corporation asked scientists at NIST to make their own independent determination of the EUVL resist sensitivity to validate the results. Measurements conducted at the NIST SURF III Synchrotron Ultraviolet Radiation Facility agreed with those of the Berkeley group. The fact that the photoresist is now known to be twice as sensitive to the EUV light implies that half as much light energy as had been expected is arriving at the wafer.

"These results are significant for a technology that faces many challenges before it is slated to become a high-volume manufacturing process in 2012," Grantham says. "It should open the eyes of the industry to the need for accurate dose metrology and the use of traceable standards in their evaluations of source and lithography tool performance."

* S. Grantham, C. Tarrio, R. E. Vest, T. B. Lucatorto, A. Novembre, M. Cangemi, V. Prabhu, K.W. Choi, M. Chandhok, T. Younkin and J. S. Clarke. SEMATECH EUV Source Workshop, Bolton Landing, N.Y., May 12, 2008.

Media Contact: Mark Esser, mark.esser@nist.gov, (301) 975-8735

'Electron Trappinga May Impact Future Microelectronics Measurements

Using an ultra-fast method of measuring how a transistor switches from the "off" to the "on" state, researchers at the National Institute of Standards and Technology (NIST) recently reported that they have uncovered an unusual phenomenon that may impact how manufacturers estimate the lifetime of future nanoscale electronics.

The transistor is one of the basic building blocks of modern electronics, and the life expectancy or reliability of a transistor is often projected based on the response to an accelerated stress condition. Changes in the transistor' threshold voltage (the point at which it switches on) are typically monitored during these lifetime projections. The threshold voltage of certain types of transistors (p-type) is known to shift during accelerated stresses involving negative voltages and elevated temperatures, a characteristic known as "negative bias temperature instability" (NBTI). This threshold voltage shift recovers to varying degrees once the stress has ended. This "recovery" makes the task of measuring the threshold voltage shift very challenging and greatly complicates the prediction of a transistor' lifetime.

As semiconductor devices reach nanoscale (billionth of a meter) dimensions, measuring this device reliability accurately becomes more important because of new materials, new structures, higher operating temperatures and quantum mechanical effects. Many NBTI studies show that the accuracy of the measured threshold voltage shift (and consequent accuracy of the lifetime prediction) depends strongly on how quickly the threshold voltage can be measured after the stress is finished. So, NIST engineers began making threshold voltage measurements at very fast speeds, leaving as little as two microsceconds (millionths of a second) between measurements instead of the traditional half-second interval. What they observed was surprising.

"We found that NBTI recovery not only returned the threshold voltage to its pre-stressed state but briefly passed this mark and temporarily allowed the transistor to behave better than the pre-stressed state," says Jason Campbell, a member of the NIST team (that includes Kin Cheung and John Suehle) who presented this finding at the recent Symposium on VLSI Technology in Hawaii. The NBTI effect generally is believed to result from the buildup of positive charges, he explained, but the new observations at NIST indicate the presence of negative charge as well. NIST' ultra-fast and ultra-sensitive measurements revealed that during recovery, the positive charges dissipated faster than the electrons, giving the system a momentary negative charge and heightened conductivity.

To date, Campbell says, transistor manufacturers only consider the accumulation of positive charges to predict the longevity of their microelectronics devices. "But as these systems get smaller and smaller, the electron trapping phenomenon we observed will need to be considered as well to ensure that transistor lifetime predictions stay accurate," he says. "Our research will now focus on developing and refining the ability to measure that impact."

Media Contact: Michael E. Newman, michael.newman@nist.gov, (301) 975-3025

Oxygen Ions for Fuel Cells Get Loose at Low(er) Temperatures

Researchers determined that a new material for fuel cells releases oxygen ions easily at low temperatures because many of the oxygen ions"marked here as O4"are not closely bound to the material's crystal framework.

Credit: X. Kuang, University of Liverpool

Seeking to understand a new fuel cell material, a research team working at the National Institute of Standards and Technology (NIST), in collaboration with the University of Liverpool, has uncovered a novel structure that moves oxygen ions through the cell at substantially lower temperatures than previously thought possible. The finding announced this month in Nature Materials may be key to solving fuel cell reliability issues and lead to reduced operating costs in high-performance stationary fuel cells.

Electricity is produced in fuel cells from the electrochemical reaction between a hydrogen-rich fuel and oxygen that produces electric current and water. Research on small fuel cells for cars has dominated the news, but stationary fuel cells are the Goliaths"operating at up to 70 percent efficiency and providing enough electricity"up to 100 megawatts"to power small cities, hospitals, military installations or airports without relying on the electric power grid. Smaller versions are being considered for auxiliary power units in such applications as refrigeration trucks to reduce engine idling.

They are called "solid oxide" fuel cells (SOFCs) because the heart of the cell is a solid electrolyte that transports oxygen ions extracted from air to meet with hydrogen atoms. This alchemy traditionally requires high temperatures"about 850 degrees Celsius in conventional SOFCs"and therefore long startup times, ranging from 45 minutes to eight hours.

The high temperatures necessitate more expensive materials and higher operating costs, so stationary fuel cell research is focused on lowering operating temperatures as well as shortening startup times. The U.S. Department of Energy' goal is to slash the startup time to two minutes.

Chemists at the University of Liverpool fabricated a new oxygen ion electrolyte material of lanthanum, strontium, gallium and oxygen and sent it to the NIST Center for Neutron Research (NCNR) to investigate with collaborators from NIST, the University of Maryland and University College London. Neutrons provide an atomic-scale view of materials so scientists can "see" what is happening at that level.

The oxygen ions in the new materials become mobile at 600 degrees C, much lower than previously studied materials. Researchers suspected the reason lay in the location of the oxygen ions in the crystal framework of the compound. The neutron probes allowed them to determine the basic crystal structure that held the lanthanum, strontium, gallium and oxygen atoms, however the exact nature of the extra oxygen ions was unclear.

NCNR researchers recommended borrowing a method from radio astronomy called maximum entropy analysis. "When astronomers are not able to visualize a specific part of an image because it constitutes such a small part of the total information collected, they utilize a part of applied mathematics called information theory to reconstruct a sharper image," explains NCNR researcher Mark Green. "The combination of neutron diffraction and maximum entropy analysis not only allowed us to determine the location of additional oxygen ions outside of the basic framework, but revealed a new mechanism for ion conduction."

"It allows us to take a fundamentally different approach in the design of future materials, so that we can harness this new mechanism for oxide ion conduction and produce lower operating fuel cells," says Green. "This type of work is very important to us, which is why as part of the NCNR expansion we are developing a new materials diffractometer that will greatly enhance our capabilities in energy related research."

* X. Kuang, M.A. Green, H. Niu, P Zajdel, C. Dickinson, J.B. Claridge, L. Jantsky and M.J. Rosseinsky. Interstitial oxide ion conductivity in the layered tetrahedral network melilite structure. Nature Materials, June 2008

Media Contact: Evelyn Brown, evelyn.brown@nist.gov, (301) 975-5661