Subcategory: Materials Science
Erika M. Spangler - University of the District of Columbia
Co-Author(s): Eva Mutunga, University of Tennessee, Knoxville, TN
The ability to machine materials on the nanometer scale has many applications ranging from nanoelectronics and integrated circuit editing to the creation of nanopore sensors and optical devices. The Orion helium ion microscope (HIM), with a typical probe size of less than 1 nm, offers a unique method for nanofabrication at a scale currently unattainable by conventional gallium-based focused ion beam (FIB) processing. However, the slow material removal rate (low sputter yield) and helium implantation damage impart challenges to this technique. In addition, each sample mills at a different rate depending on the material properties, sample thickness and beam energy. In this study, we seek to find optimal conditions for helium ion nanomachining of thin film and bulk substrates. Using previous results from a collaboration with Intel Corporation, which explored helium ion beam interaction with silicon substrates, a methodology for quantifying beam-sample interactions via cross-sectional imaging has been developed. This methodology was then applied in this work to study helium ion beam interaction with gold substrates in particular. SRIM (Stopping and Range of Ions in Matter) simulation software, which uses a Monte Carlo method, was employed to explore trends between sample thickness, beam energy and sputter yield (mill rate). The theoretical sputter yield reached a constant value of 0.11 Au atoms per incident He ion at thicknesses great than 200 nm with a landing energy of 35 keV. Experimental sputter volume at varying ion doses was measured using crosssectional TEM images of a 110 nm gold foil after being exposed to 35 keV helium ions at doses ranging from 1×105 to 1×106 ions/nm. From these images it was clear that two types of sputtering occurred during the experiment: backward sputtering and forward sputtering. Initially the 110 nm sample acts a bulk sample where only backward sputtering is present; however, as the sample begins to thin the transmitted ions maintain enough energy to forward sputter the opposite side of the foil. We found that as sample thickness decreased, backward sputter yield also decreased – a trend that agrees with the SRIM model to within 10%.
Although this research has confirmed the relationship of sample thickness and backward sputter yield, an improved model would include the effects of forward sputtering. Future aspects of this research would include repeating the gold foil experiment with a linearized dose scale to obtain higher accuracy in our mill rate calculation and compare these results with bulk gold. Furthermore, we would like to experimentally measure the effect of landing energy on the sputter yield; optimizing these parameters will provide optimal nanomachining parameters using the lowest dose feasible.
Funder Acknowledgement(s): This study was supported, in part, by a grant from NSF ATE Program awarded to Kate L. Klein, PhD, School of Engineering and Applied Sciences, University of the District of Columbia, Washington, DC 20008. Additional support is from the National Institute of Standards and Technology Summer Undergraduate Research Fellowship (SURF) Program.
Faculty Advisor: Kate L. Klein,