At AVS/AVEM: Bio-sensors, SAMS, quantum dots and more

By Debra Vogler

WaferNews Technical Editor

Of the numerous topics covered in technical sessions at the recent American Vacuum Society (AVS) conference in San Francisco, arguably one of the timeliest was the matter of self-assembled monolayers (SAMs) – already being used for bio/chemical sensors.

According to U. Washington Assistant Prof. S. Jiang, who presented a paper he co-authored on SAMS, “In order to reach a very low detection limit and detect many compounds at the same time (multi-channel sensors), one has to control surface properties at the molecular level.

“We are able to control and characterize surface chemical and structural properties of self-assembled monolayers at the molecular level. Based on the platforms, we control and manipulate the immobilization of antibodies on surfaces and detect antigens (e.g., biological warfare agents) at a very low detection limit,” he explained.

Jiang currently leads a DARPA project aimed at developing molecular engineering of surface sensing and detection. “This research is more fundamental,” states Jiang. “Therefore, the success of this project will have quite a broad impact, besides improving the performance of many types of sensors.” Sinclair Yee, professor of electrical engineering at U. Washington, leads a US Army project to develop portable sensors for field applications.

Another research project between the U. Heidelberg and Hitachi’s Central Research Laboratory will also have an impact on the fabrication of quantum dots, optical switches, mesoscopic lasers and biosensors.

Currently, Prof. Michael Himmelhaus from U. Heidelberg reports that his team can control adsorption on distinct sites of chemically modified surfaces. “We apply micro contact printing just for convenience, any other method should work as well,” explains Himmelhaus. “This facilitates fabrication because the method is very easy to control and particles only stick where they should. Particles adsorb in a random, close-packed fashion, but this might not be sufficient for nanotechnology where PS particles serve as templates for regular arrays of nano-dots. However, for other applications, such as micro-optics and life sciences, irregular monolayers are just fine. Two-dimensional lenses and gratings with dimensions of several microns can be readily fabricated by our technique.”

In the future, Himmelhaus sees two directions. The first is achieving two-dimensional crystallization within the adsorption sites. “Matching such a superlattice spacing with the size of the particles might lead to new photonic band-gap structures and devices, such as optical switches and lasers,” says Himmelhaus.

The second direction is the utilization of linker molecules to bind a distinct number of biomolecules to the PS spheres. “Currently, dosing is still one of the major problems in the commercialization of biosensors and related devices,” he reports. “With our method, it might become possible to control the number of surface active biomolecules at distinct locations of the surface very precisely.”

Advances in bio and medical sensor technology are being enabled by a new type of measurement tool – a quartz crystal microbalance with dissipation monitoring (QCM-D) by Q-Sense, Gothenburg, Sweden. According to President Patrik Dahlqvist, QCM-D provides data on the mechanical properties of surfaces such as elasticity and viscosity that can’t be obtained with classical QCM techniques. “The technique works in liquids, bio-molecules and soft films.” Although not originally thought to be directly useful in IC production, Dahlqvist noted that one of the company’s customers, Rodel, is working on using the technology to study how etching fluids are working from kinetics and chemistry viewpoints.

Basic scientific research proved to be a major draw as the room filled up shortly before R. Stanley Williams, of HP Labs, gave a presentation on epitaxial growth of self-assembled dots and wires. Williams noted that when he began studies on quantum dots, people asked him what he was going to do with them. At the time, he admitted, he didn’t know. But making a great leap of faith, he believed that if he could understand the fundamental growth mechanism of Ge dots on a Si surface, he could controllably grow wires, and he knew what to do with those.

Describing the principles of what he calls nano-architectonics, Williams explained that crystal symmetry primarily determines the shape of the structures grown, surface energy (which can be controlled using surfactants) primarily determines their size, and spacing determines strain fields. He then presented results achieved using a $3M CVD production tool, as opposed to an academic system, which he said could not have accomplished the task. Pyramids, domes and superdomes of Ge were formed on Si(001). The temperature of the system and amount of Ge on the surface changes interactions between islands, which interact via strain fields.

The end result of Williams’ research was the growth of a single ErSi2 nano-wire on Si(001), nine atoms wide. He also showed parallel DySi2 wires with a 9nm +/- 1nm separation that were many microns long. Williams said the aspect ratios were fantastic and the uniformity excellent. HP Labs is now in the process of examining the electrical properties of these wires (e.g., tunneling current). The conductivity of the wires appears to be quantized, although these results are preliminary.

Unfortunately, basic research in physics and chemistry, while exciting, doesn’t appear to be capturing the imagination of American youth. Rudolf Ludeke, Ph.D., president-elect of the AVS and research staff member at IBM’s T.J. Watson Research Center, lamented this in his Association of Vacuum Equipment Manufacturers (AVEM) address. Noting that fewer Americans are entering our universities to major in the physical sciences, the basis for all technical innovation, he cited some unnerving statistics.

“Bachelor degrees in physics are at a 40-year low, comprising but 0.3% of all bachelor degrees granted by American colleges in 1999,” said Ludeke. “Our physics Ph.D. production continues to decline, with less than 1,300 degrees granted in 1999. More concerning is the fact that the majority of these are not US citizens, many of whom will opt not to enter the US labor force.”

“We at the AVS, and I, will make it a goal for the coming year to promote a greater awareness on [Capitol] Hill of scientific issues affecting our community,” stated Ludeke. “As an organization we have endorsed in the past various statements promoting increased federal funding in research and education. But we must go beyond this. We will enhance the effectiveness of our participation at the Congressional Visit Days. We will increase our awareness of pending, science-impacting legislation, and we will co-sponsor a congressional science fellow.”


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