Images Of Landau Levels
For the first time, researchers have imaged the phenomenon of Landau levels, the quantum levels that determine electron behavior in a strong magnetic field. Nobel Prize winner Lev Landau conceived of this, in theory, back in 1930.

Using scanning tunneling spectroscopy, a spatially resolved probe that interacts directly with the electrons, the University of Warwick and Tohoku University have discovered internal ring-like structures of these Landau levels at the surface of a semiconductor.

Scanning tunneling spectroscopy is used to study the real-space local density of states of a two-dimensional electron system in a magnetic field. By Fourier transforming the local density of states, researchers found a set of n radial minima at fixed momenta for the nth Landau levels.
“On the face of it this might seem far removed from everyday life,” said Rudolf Roemer, professor of the Department of Physics at the University of Warwick. “However the question of what defines a kilogram is currently being debated, with the spacing between the rings of these Landau levels acting as a kind of marker for a universal weight. So next time you measure out your sugar to bake a cake, you might unknowingly be making use of these quantum rings.”

Monitoring Etches In Real Time
The University of Illinois at Urbana-Champaign has devised a new method for etching circuits.

Researchers have developed an epi-diffraction phase microscopy (epi-DPM) technology. This is a non-destructive optical method for monitoring semiconductor fabrication processes in real time and with fine sensitivities. This could one day enable engineers to better monitor and control the performance properties of their fabricated devices.

With its technology, researchers took 3D images of an etched gallium-arsenide device. The topographic images are obtained from a single camera exposure. It also uses a compact Mach–Zehnder interferometer, which generates quantitative amplitude and phase maps of a field size. The unit renders topography information with 2.8nm spatial point-to-point and 0.6nm temporal frame-to-frame sensitivity.

In effect, the camera films a 3D movie “showing the dynamics of wet etching and thereby accurately quantify non-uniformities in the etch rate both across the sample and over time,” according to researchers. By displaying a digital image on the sample with a computer projector, the University of Illinois performed photochemical etching to define arrays of microlenses. It also monitored the etch profiles with epi-DPM.

“You can use light to image the topography and you can use light to sculpture the topography,” said electrical and computer engineering professor Gabriel Popescu, on the university’s Web site. “It could change the future of semiconductor etching.”

Lynford Goddard, electrical and computer engineering professor at the University of Illinois, added: “The idea is that the height of the structure can be determined as the light reflects off the different surfaces. Looking at the change in height, you figure out the etch rate. What this allows us to do is monitor it while it’s etching. It allows us to figure out the etch rate both across time and across space, because we can determine the rate at every location within the semiconductor wafer that’s in our field of view.”

Microwave Scopes To The Rescue
The National Institute of Standards and Technology (NIST) has devised a new and more advanced near-field scanning microwave microscope (NSMM).

The instrument will allow researchers to explore the properties in various materials. Using existing commercial and home-made NSMMs, NIST has been able to conduct research in molecular electronics, carbon nanotubes, nanowires, graphene and spin-based electronics.

The new instrument is expected to accelerate those efforts. A NSMM consists of an atomic force microscope (AFM) combined with a continuous or pulsed microwave signal applied to the AFM tip. NSMMs can be tuned to produce images at depths ranging from sub-micrometer to 100 microns below the surface.

Existing instruments have a single tip that is open to the air. The new NSMM has four tips, permitting simultaneous comparisons of materials. It is enclosed in an ultra-high vacuum chamber to minimize signal interference and sample contamination.

“Basically, what we’re doing is using the very fine spatial resolution of scanning probe instruments such as scanning tunneling microscopes or atomic force microscopes (AFM) and combining it with the broadband compatibility of microwave measurements,” said Mitch Wallis of the Radio-Frequency Electronics Group at NIST, on the agency’s Web site.

“Our motivation is that we want to look at things like magnetic resonance or mechanical resonance on the nanoscale using microwaves. If you look at your cell phone or your computer, they’re all operating in the range of a few gigahertz. So we have to measure the nanoscale objects that make up those devices to get an understanding of how they perform at those frequencies. Otherwise, it’s going to be much harder to integrate them into useful commercial devices,” he added.

Faster Spike Anneals
Sponsored by the Semiconductor Research Corp. (SRC), Cornell University has developed a new laser-based method for fast anneal of thin photoresist films used to transfer semiconductor patterns onto silicon wafers. The new laser-based approach provides improvement in the critical lithographic steps required for creation of integrated circuits.

Photoresists are applied to the wafer surface to form thin, patternable layers. Historically, a lengthy bake of the wafer at low temperatures has been required to avoid degradation of the photoresist properties.

Currently, they are exposed and then baked on a hot plate at low temperatures of 80-150°C (175-300°F) for approximately one minute to activate the resist chemistry and create a solubility differential between exposed and unexposed parts of the resist, which delineates the post-develop pattern. The photoresist typically consists of a proprietary mixture of polymers formulated for specific lithography treatments.

Prolonged heating, especially at higher temperatures, causes excessive chemical diffusion to take place, which degrades the image quality. However, Cornell’s novel application of laser spike heating, which lasts for a duration of milliseconds, preserves the polymer integrity at much higher temperatures up to 800°C or 1450°F.

It provides a means to maximize resist sensitivity while minimizing pattern roughness, thereby facilitating enhanced scalability. Cornell has determined that heating at much higher temperatures for millisecond times using continuous wave lasers not only activates the necessary photoresist chemical reactions at higher throughput, but also improves the pattern fidelity and line-edge roughness over conventional methods.

“Faster, higher fidelity pattern transfer in the fab means better chip performance at reduced cost. This new laser method can deliver a breakthrough in thermal processing for the industry,” said Christopher Ober, professor of materials science and engineering at Cornell, on SRC’s site. “Until now, lithography progress has been held back by the traditional methods for heating the resist that were regarded by many as already optimized. The laser proves otherwise.”

Tags: atomic force microscope, Cornell University, eli-diffraction phase microscopy, Landau levels, National Institute of Standards, NIST, Semiconductor Research Corp., Tohoku University, University of Illinois, University of Warwick

This entry was posted on Tuesday, October 2nd, 2012 at 12:01 am and is filed under News Stories. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.