Analytical Chemistry Seminar Series

Seminars are Wednesdays at 11¹⁵ AM in S5 Osmond unless otherwise noted.
Refreshments are served at 11 AM.

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The Scanning Capacitance Microscope (SCM) combines a high-sensitivity capacitance measurement with an atomic force microscope (AFM). When applied to semiconductors, the SCM measures a differential capacitance signal that is related to the local carrier concentration beneath the tip. NIST has had an effort to develop the SCM since 1993. This talk will review the principle of operation of the SCM, practical metrology aspects of measuring SCM images, models of the SCM measurement, and techniques for extracting carrier profiles from SCM images. SCM images of cross-sectioned 0.18 mm MOSFETs and examples of extracted 2-D carrier profiles will be shown.

The Semiconductor Industry Associations’ (SIA) National Technology Roadmap for Semiconductors identifies two-dimensional carrier profiling as a key enabling technology for the development of next-generation integrated circuits. In 1999, it is desired to know 2-D carrier profiles with spatial resolution of 5 nm and with a precision (in concentration) of 5%; these demands increase to < 1 nm and 2% by 2015. The SCM has emerged as the leading contender to provide 2-D carrier profiles.

To continue the ongoing electronics revolution well into the twenty-first century, it is essential that devices and circuits be miniaturized down to the nanometer scale. Two broad approaches exist for achieving such "nanoelectronics." One approach, solid-state nano-electronics, is attempting to sculpt smaller and smaller features on solid-state semiconductor surfaces in order to manufacture denser computer chips. However, this approach is becoming ever more difficult and costly as miniaturization progresses. A promising alternative approach which may be less costly is to use natural nanometer-scale structures--i.e., individual molecules--to make the electronic components [1].

Molecules can be made precisely, identically, and cheaply in enormous numbers. Moreover, during the past several years there has been great progress in the development and the demonstration of such "molecular electronic" devices, individual molecules that conduct and switch electrical currents.

The speaker will review and explain these recent experimental results that are establishing a foundation for building tiny powerful computational and control systems integrated on the molecular scale. Further, he will describe research at The MITRE Corporation that is building upon these experimental results to propose detailed designs for molecular electronic digital logic circuits and functions. All of these detailed logic designs include only experimentally demonstrated molecular electronic devices as their components [2].

One of these molecular electronic circuit designs describes a molecule that adds two numbers when a current is passed through it. The structure of this molecular electronic half adder is depicted in Figure 1. In the figure, A and B represent the one-bit binary inputs to the adder, while S and C represent the one-bit outputs, the sum and the carry bits, respectively. The corresponding conductive molecules, if realized, would use much less power and they also would be as much as one million times smaller in area than the comparable circuits on a state-of-the-art commercial microcomputer chip [2].

[1] D. Goldhaber-Gordon, M. S. Montemerlo, J. C. Love, G. J. Opiteck, and J. C. Ellenbogen, "Overview of Nanoelectronic Devices," Proceedings of the IEEE, vol. 85, no. 4, April 1997, pp. 521-540.

[2] J. C. Ellenbogen and J. C. Love, "Architectures for molecular electronic computers: 1. Logic structures using molecular electronic diodes," Report MP 98W0000183, The MITRE Corporation, McLean, VA, July 1999. This report soon will be available on the Internet at the URL: http://www.mitre.org/technology/nanotech

We have studied the etching of Si(100)-2x1 by Cl and Br, using scanning tunneling microscopy to obtain morphological information that can be related to reaction and desorption pathways. Clean surfaces were exposed to molecular halogens at room temperature to produce well-defined chemisorption structures for coverages in the range 0.2-1.0 ML. Heating to 750-850 K induced etching by thermal desorption. Analysis of the halogen concentration before and after heating indicated that the rates of desorption for SiCl2 or SiBr2 were greatest for intermediate coverages and that etching was suppressed as saturation was reached. Hence, desorption is not simply proportional to the concentration of species that can form adsorbed precursors SiX 2(a). Instead, it is directly coupled to the creation of monomer vacancies adjacent to the SiX 2(a) unit because this increases the lifetime of the excited state and increases the likelihood of its desorption. Increasing the surface concentration of halogens reduces the rate of vacancy formation. We show that these rates are also affected by a redimerization process in the high temperature Br-stabilized Si(100)-3x1 reconstruction that increases the likelihood of SiBr 2(a) formation and enhances its desorption. I will also discuss recent results for F etching on Si(100)-2x1.

Surface and interfacial chemistry plays a critical role in many material systems. The speaker will focus upon work in his laboratory that concerns corrosion and oxidation of metal and composite systems. A variety of experimental approaches will be discussed, with emphasis placed upon the role of valence band photoemission and the use of experimental approaches specially adapted to material systems. An apparatus will be described that allows studies to be made of the solid-liquid interface. The apparatus is linked to an X-ray photoelectron spectrometer (XPS), equipped with monochromatized X-rays, so allowing an investigation of the changes in surface chemistry at this interface. Another apparatus will be discussed that allows carbon fibers and composites containing carbon fibers to be examined at high temperatures and under simulated oxidation conditions. The value of using valence band XPS interpreted by calculation models will be demonstrated for these systems, and the use of core and valence band XPS for the study of composite interfaces will be demonstrated. Examples discussed will include the corrosion and oxidation of nickel, molybdenum, aluminum and iron, and the importance of surface oxidation in the production of useful composites containing carbon fiber reinforcement.

This talk will focus on the measurements of methane found in core samples of methane hydrates found buried ~2 km deep beneath continental shelves and tundra. The problem is that the methane escapes the core sample before they can reach the surface, therefore remote sensing and analysis needs to be done.

Other outside speakers yet to be scheduled:
Prof. Raoul Kopelman, University of Michigan
(Host: Anne Andrews, x5-2970 )