Improved AFM Data for Nanoelectronics Research Using Negative-Stiffness Vibration Isolation

Nanoelectronics came into use long after 1977 though Dr David K Ferry was already engaged actively in developing some of the smallest transistors of the world. The field, which at that time was known as ‘ultra small devices’, was in its infancy and Dr. Ferry's research team was one of only four select groups researching the limits of small electronic devices.

Today Dr Ferry leads the Nanostructures Research Group at Arizona State University (ASU) in Tempe, which is a collection of faculty, staff and students working on research in the regimes of nanolithography, the physics of nanostructures and ultra-small semiconductor devices.

The group belongs to the University's College of Engineering, Center for Solid State Electronics Research, whose alumni make up a serious constituency throughout the nanoelectronics universe in both industry and academia. Their present interests lie in the area of quantum dots, quantum wires and ultra-small semiconductor devices in a variety of materials.

A wide array of theoretical studies of quantum transport was conducted by the group in these very small devices. For instance they are involved in a process known as scanning gate microscopy at low temperatures.

The system is mounted in a large cryogenic cooler, which is an enclosed container with a helium-3 cooling system, an isotope of a helium molecule-which is brought down to 300 milli-Kelvin, or 1000 times below room temperature, about one-half a degree above absolute zero.

The cooler is enclosed with a vacuum jacket so the heat can't transmit in, and it prevents the cold from being mitigated by the ambient room temperature. Typically with the AFM, one moves the cantilever along the surface and then notes the change in position as it goes over topography on the surface.

The team of Dr Ferry is using a process known as a piezo-electric sensor in which the AFM cantilevered tip is metalicized with a very thin layer of metal so that a voltage can be applied to it. As the tip moves it creates a voltage across the plane, which is measured to determine certain mechanical property values. This is a technique that was developed four or five years ago at Harvard University.

High Level of Vibration Isolation Required

During measurement of very few angstroms or nanometers of displacement, an absolutely stable surface upon which to rest the instrument is critical. If the surface is not stable, any vibration coupled to the mechanical structure of the instrument will cause vertical noise and basically an inability to measure these kinds of high resolution features.

Dr Ferry added that any kind of vibration noise in the system causes movement in the AFM cantilever tip and incorrect data and bad signals are obtained. Dr Ferry went further when compared to most university applications as they integrated a large magnet into the system something that for instance Harvard is putting into their operation.

The magnet helps see different types of transport. The magnet can be turned on and the magneto transport of semiconductors can be seen. It is a quite a different mode of transport altogether. Figure 1 shows ASUs Dewar sitting on negative-stiffness vibration isolation tables and Figure 2 shows the top section of the Dewar.

Figure 1. ASUs Dewar sitting on Negative-Stiffness vibration isolation tables.

Figure 2. Top portion of the Dewar.

The entire system not just the cantilever needs had to be isolated. An extremely high level of vibration isolation is needed given research parameters. Modern electronic devices from the experiments are being derived by the team.

Dr. Ferry's Research Covers:

• Electron beam lithography of quantum dots and quantum devices with applications that include quantum ballistic transport at very low temperatures and high magnetic fields, as also the quantum-classical transition and the role of quantum effects in real devices at room temperature.
• Magneto-transport studies used to probe the nature of electron dynamics in semiconductor quantum dots, which are quasi-zero-dimensional structures whose size is comparable to the Fermi wavelength of the electrons themselves. Figure 4 shows the AFM image of a quantum point contact showing isolation benches.
• Surface chemical analysis done using a scanning Auger microprobe. Under good conditions, a lateral resolution of about 25 nm can be achieved.
• Professor Michael Kozicki, in the group, has examined Chemically Enhanced Vapor Etching (CEVE) patterning technique. He has used hydrocarbon contamination layers from laboratory air or vacuum chamber ambients and successfully demonstrated nanoscale pattern formation in silicon dioxide.
• A nitrogen chamber coupled directly to a UHV STM/AFM facility for CEVE processing of silicon dioxide resists, and their use in semiconductor device fabrication has also been developed. Within the nitrogen chamber there is a processing system for the actual CEVE development.

Figure 4. AFM image of a quantum point contact showing isolation benches.

Negative Stiffness Isolators

The main benefit of a negative-stiffness isolator is that it is not powered it has no electricity going to it. So, in a site where heat buildup can be an issue, such as with enclosed cryogenic chambers, negative-stiffness becomes a highly efficient option.

Negative-stiffness isolators use a mechanical concept in low-frequency vibration isolation. Vertical-motion isolation is provided by a stiff spring that supports a weight load, combined with a negative-stiffness mechanism (NSM). Beam-columns connected in series with the vertical-motion isolator provide horizontal-motion isolation. The horizontal stiffness of the beam-columns is reduced by the "beam-column" effect.

The capability offered by negative-stiffness isolators is quite unique to the field of nanotechnology, specifically, the transmissibility of the negative-stiffness isolator, that is, the vibration that transmits through the isolator as measured as a function of floor vibrations, which is substantially improved over active isolation systems. Even though active isolation systems fundamentally have no resonance, their transmissibility does not roll off as fast as negative-stiffness isolators. Figure 5 shows negative stiffness isolation performance by transmissibility

Figure 5. Negative stiffness isolation performance by transmissibility.

Conclusion

In comparison with other laboratory research instrumentation, the growth of AFM usage has been quite extensive over the past 10 years. Since its inception in 1988, it has continuously proven to be a key tool in moving nanotechnology research forward.

Today, the transistors have critical dimensions down around 25 nanometers. And the most critical dimension is the oxide thickness, which is 1 nanometer. When you consider that you have to control one nanometer vertical thickness over 300 millimeters of lateral dimension that is a difference of 10 to the 8th power. The need for effective vibration isolation has never been greater, and will continue to become more demanding as the nano-industry progresses.

About Minus K Technology

Minus K^® Technology, Inc. was founded in 1993 to develop, manufacture and market state-of-the-art vibration isolation products based on patented negative-stiffness technology. Minus K^® is based in the Los Angeles area.

The Minus K^® products, formerly sold under the trade name Nano-K^®, represent an important enabling technology. By reducing building and floor vibrations to unprecedented levels these systems enable vibration sensitive instruments and equipment to perform at unprecedented levels.

They are used in a broad spectrum of applications including nanotechnology, biological sciences, semiconductors, materials research, zero-g simulation of spacecraft, and high-end audio. Minus K^® is an OEM supplier to leading manufactures of scanning probe microscopes, micro-hardness testers and other vibration-sensitive instruments and equipment. Minus K^® customers include private companies and more than 200 leading universities and government laboratories in 43 countries.

Dr. David L. Platus is President and Founder and is the principal inventor of the technology. He earned a B.S. and a Ph.D. in Engineering from UCLA, and a diploma from the Oak Ridge School of (Nuclear) Reactor Technology. Prior to founding Minus K^® Technology he worked in the nuclear, aerospace and defense industries conducting and directing analysis and design projects in structural-mechanical systems. He became an independent consultant in 1988. Dr. Platus holds over 20 patents related to shock and vibration isolation.

This information has been sourced, reviewed and adapted from materials provided by Minus K Technology.

For more information on this source, please visit Minus K Technology.