Plasma-Surface Interactions During In-Situ Photo-Assisted Etching

University Of Houston, Houston TX

Investigators

Abstract

Reactive ion etching (or plasma etching) is a critical operation in the manufacturing of integrated circuits (or 'chips') to produce extremely precise features, at the nanometer (a billionth of a meter) scale. Without this process we would have no portable cell phones, laptop computers and all the other modern marvels we take for granted. Ever smaller feature dimensions will allow more transistors to be packed onto chips, resulting in increased information storage and faster computers. This evolution in chip speed and function has progressed steadily over the past 50 years, following Moore's Law, which states that the number of transistors on a chip doubles about every 18 months. Recently, while studying reactive ion etching of silicon, a rather startling discovery was made: silicon was etched, even when no reactive ions were present. Careful experiments revealed that this etching was due to photons, originating in the plasma. This in-plasma photo-assisted etching produced etched feature shapes that are not optimum for integrated circuits and thus may prove to be a show-stopper in the burgeoning field of etching with atomic precision. This project will combine experiments and simulations to understand the mechanism of in-plasma photo-assisted etching, and identify conditions to suppress this phenomenon. The study will produce a fundamental understanding of the photo-physics and chemistry at the plasma-semiconductor interface and will have a significant impact in the microelectronics industry as well as the field of nanotechnology, with clear societal benefits. This systematic investigation of plasma-surface interactions will focus on in-situ photon-plasma synergism, and its effect on etching of semiconductor materials in halogen-containing plasmas. A combination of experiments and simulations will address questions such as: (a) What are the synergistic effects of photons and i) positive ions, ii) electrons, iii) negative ions, iv) halogen atoms? (b) does the sheath potential affect photo-assisted etching rates and if so, how and what is the cause? (c) what is the effect of surface plasmons (plasmonics) in the case of samples patterned with sub-wavelength features? A novel dual plasma reactor will be employed, to provide controlled fluxes of ions, UV-VUV photons, and radicals bombarding the substrate. The UV-VUV light intensity, ion flux and ion energy striking the substrate will be measured through a pinhole on the substrate holder, in a differentially pumped analysis chamber. Self-consistent simulations of the electric field distribution and species (electrons, holes, positive ions) fluxes in the plasma as well as in the solid, and electromagnetic calculations of surface plasmon propagation and absorption will be performed. Simulations coupled with experiments will provide insights in the mechanism of photo-assisted etching for varying substrate bias (potential of the sheath over the semiconductor), photon flux and energy, dopant concentration, as well as nanofeature size and aspect ratio. Finally, attempts will be made to exploit photo-assisted etching of silicon to create nanoholes with dimensions (e.g., 3 nm dia.) much smaller than those produced by conventional reactive ion etching.