The goal of this research project is to improve the resolution and writing speed of "nanopantography", to make it a manufacturing-worthy process. This research will make it possible to manufacture any desired nanopattern, in a variety of materials, over large areas, with relatively short processing times. It will build on a "nanopantography" method that has been successfully demonstrated. This method uses a large area ion beam, directed at an array of electrostatic microlenses fabricated on a substrate. By applying appropriate voltages to the microlenses and "rocking" the substrate in a controlled fashion, "beamlets" entering the lenses focus to spots that can be rastered across the substrate in a massively parallel fashion, allowing patterns to be written quickly at beyond state-of-the-art resolution of 5 nanometers or less. Improvements will result in a 300 mm-diameter full wafer processing time of ~30 mins, a relatively high throughput, considering that there would be only a few steps requiring ultra-high resolution nanopatterning in a device fabrication process.
This work will provide challenging projects for two PhD students and two undergraduates, with rich scientific and educational payoffs, as well as technological advances. Basic knowledge will emerge on nanofabrication, advanced plasma sources, and ion-surface interactions. The project will have broad societal impact on diverse areas of nanotechnology, including microelectronic devices, bio-sensors, and displays. Outreach activities will increase public awareness for societal benefits of plasmas applied to nanotechnology. As part of this effort, undergraduate students and a high school teacher will be involved in the project.
The sustained evolution of ever increasing functionality has pushed the semiconductor industry to continue to reduce the size of transistors and interconnecting wires that make up the integrated circuits, that are at the heart of all smart phones, tablets and laptop computers, as well as desktop and main frame computers, microprocessors in all modern automobiles and aircraft, medical diagnostic equipment, video games and countless other devices. The current technology uses a photo-printing technique, known as photolithography, and various other "tricks" to allow "critical dimensions" of 20 nanometers (5000 times smaller than the diameter of a human hair) to be printed. Within a decade, photolithography will no longer be capable of achieving the required dimensions of less than 10 nanometers that will be called for in this technology. No other manufacturing-worthy alternative has emerged as the obvious successor to photolithography. Techniques capable of producing smaller dimensions, including x-ray and extreme ultraviolet lithography, direct writing by electron beam lithography or ion beam lithography, imprint methods and self-assembly, all have serious drawbacks. Other devices that are yet to be produced or even imagined will require creating structures with dimensions of a few nanometers. The work carried out in the present study built on prior work from this laboratory. A direct writing method, dubbed "nanopantography", was further developed to produce features as small as 3 nm, smaller than those produced by any other method. The nanopantography method uses a broad area ion beam with a well-defined energy, extracted from a pulsed plasma source. The beam impinges on the substrate that contains arrays of microlenses (holes in a metal/insulator bi-layer covering the substrate) that focus the beamlets entering each lens to a fine point. Material is removed at these focus points. By tilting the substrate, the focus points are moved across the bottoms of the lenses, allowing any chosen pattern to be written simultaneously in each lens. The method is capable of producing many (billions) of identical structures simultaneously over large areas (many square centimeters). The throughput in the present study was greatly improved by using a two-step "amplification" process in which nanopantography was used to write a very shallow pattern into the native oxide layer on silicon, and then this pattern was transferred deep into the silicon by highly selective etching in a chlorine plasma. This plasma etching process was carried out in a standard commercial machine used to manufacture integrated circuits. A unique feature of the plasma transfer process is that it is based on photon-initiated etching as opposed to ion-assisted etching, thereby allowing an ultrathin oxide layer on the silicon substrate to act as an etching mask and offering a much less damaging process for pattern transfer. The advanced ion beam source depicted in Figure 1 was developed as part of this work. It produces a beam of ions with a very narrow energy spread at relatively high intensity, and will likely find other applications. It was incorporated into the system shown in Figure 2, which included many improvements over the previous system. The nanopantography process is schematically represented at the top of Figure 3. Some sample results are shown in the lower half of Figure 3. The grant supported a graduate student (Siyuan Tian), and (partly) a post-doctoral associate (Se Youn Moon). Siyuan Tian is in the final stages of his PhD dissertation. He has accepted a job offer from Lam Research in Fremont, CA. Dr. Moon is currently an Assistant Professor in Chonbuk University, Jeonju, South Korea. The student and post-doctoral associate working on the project were trained in the critical areas of nanopatterning, plasma source design, plasma diagnostics, and plasma science and engineering in general. In addition, our laboratory has hosted high school teachers every summer, as well as REU students. These participants were immersed in the activities in our group, including the Nanopantography project supported by this grant. In the summer of 2012, Shoshauna Harisson (black female) worked in our lab as an REU student, studying nanopatterning. She is currently a student in Chemical Engineering at UT, Austin. Also in 2012, Mr. Jarrod G. Collins (African American), a Klein Forest High School Chemistry Teacher, spent the summer with us as part of our Research Experience for Teachers (RET) program and developed a module on lithography to present to his class. For the summer 2013 we had Eduardo Hernandez (Hispanic) from the Polytechnic University of Puerto Rico as an REU student to work on plasma surface interactions, and Meghan Keefe for the RET program. Megan is teaching chemistry and physics at Cristo Rey Jesuit high school in Houston. In addition, we hosted two high school students who were trained in doing simulations of ion trajectories in electrostatic lenses of the kind used in nanopantography. Seven articles acknowledging support of this grant have been published in referred journals and two more manuscripts are in preparation.
