Condensed Matter Seminar

Massood Tabib-Azar
Electrical and Computer Engineering Dept and Bioengineering Dept, University of Utah

Tuesday, April 28, 2009; 4:00 pm in JFB 334

Nanoscale Deposition and Etching of Silicon Quantum Dots Using Field-Assisted AFM-Based CVD

Near-field microwave can be used for depositing materials [1] in addition to their well developed applications in sensing and imaging [2]. Here we report for the first time electric-field assisted decomposition of gas molecules near a conducting AFM tip to directly deposit, etch and pattern nanometer-scale silicon structures. In our approach, near-fields are used to draw in and decompose SiCl4 molecules for depositing Si, and to use SF6 molecules for etching. Decomposition of gas molecules near the AFM tip is accomplished using a 10-50MHz signal. The deposition and etching rates varies with gas flow rate, power and the type of the tip. Deposition and etching from gas phase is relatively clean process specially when the by-products are also in the gas phase. It is also potentially very fast since gas molecules at 1 atmosphere have around 60 nm mean-free-path and are readily available on demand. Owing to relatively large curvature of our current AFM tip apex ~ 20 nm, the smallest feature size we could pattern was around 30 nm. We discuss kinetics of formation of these dots and propose a reaction mechanism that is being evaluated to improve the spatial resolution of this unique technique. We also discuss the role of thermal decomposition that can deteriorate the spatial resolution of the tip-based CVD processing. To avoid thermal contribution and spreading, the tip voltage should be pulsed. We are currently adding electrodes directly on the AFM probe to eliminate any need for conducting substrate. With excitation completely contained in the tip, the AFM-based nanofabrication will become versatile and can be used as a 3-D mask-less nano-fabrication on any material from polymers to diamond to clays to bio-materials.

Acknowledgments: This work is supported by DARPA's Tip-Based Nanomanufacturing program under Dr. Thomas Kenny. We acknowledge useful discussions with Profs. Mortom Litt, Carlos Mastrangelo, Mohan Sankaran and Steve Garverick. Mr. Andrew Barnes, Dr. O. Lucian Vatamano, Mr. M. Abdlatif, Mr. Mousa Souare, and Mr. Joseph Zarycki have contributed at different stages of this work.

1. M. Tabib-Azar and M. Litt, "Silicon wafer-scale micro-fabrication factory using scanning probe micro-robots." 5th Foresight Conf. on Molecular Nanotechnology, Palo Alto, CA, November 5-8, 1997.
2. Massood Tabib-Azar and Yaqiang Wang, "Design and fabrication of scanning near-field microwave probes compatible with atomic force microscopy to image embedded nanostructures." IEEE Transactions on Microwave Theory and Techniques v 52 n 3, p.971-9 (2004).