This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

Objective: The ability to control and manipulate matter at a few tens of atoms allows researchers to investigate a regime between the atomic-scale where quantum mechanics dominate and the bulk-scale. The availability of this capability is critical to the success of both academic and industrial researchers who are making devices or studying physical effects at these scales. This project addresses this need through the acquisition of a high resolution electron-beam lithography system.

Intellectual Merit: The University of Minnesota is conducting leading edge research in magnetics, mesoscale superconductivity, nanoelectronics, nano electromechanical systems, quantum dot devices, nano scale imaging, surface enhanced Raman, and other areas. All of this work is critically dependent on the ability to write extremely fine lines. This proposal seeks funding for the acquisition of a 100 keV field-emission electron-beam lithography system for the Minnesota Nano Fabrication Center (NFC). NFC supports a large and growing group of users with innovative projects from across the nation and has proven its ability to operate and maintain complex systems for its user base.

Broader Impacts: Minnesota is a key node in NSF?s National Nano Infrastructure Network with more than 600 users last year. Enhancing graduate education of these researchers will be a major impact. NFC is also a central hub for Nano-Link, a regional center for nanotechnology AAS education. The proposed tool will be used as part of the Nano-Link effort to demonstrate cutting edge pattern definition and quantum effects that can be readily understood by technical education professionals.

Project Report

The University of Minnesota was awarded $1,389,500 for the acquisition of a high resolution electron beam lithography system. A matching award of $595,500 was also provided. A group of faculty representing electronics, optics, and magnetics, evaluated three vendors. Vistec, not only had an exceptional system, they provided a unique opportunity. They had a EBPG5000+ system on the floor that had been built for another customer, but that order was cancelled. The result is a tool capable of writing sub-8 nm lines that can operate at 50 MHz that was delivered much sooner than usual. Installation time, from tool delivery to final sign-off and initial user training, was approximately two weeks. Overall, the entire installation process proceeded without major issues of any kind, with the system passing all acceptance tests on the first attempt. The raw resolution of the tool was tested during acceptance, using an exposure pattern of extremely fine lines written in a very thin film of hydrogen silsesquioxane resist. While the final resolution of this process is dependent on resist properties as well as the tool used for exposure, the fact that the system was able to write 6 nanometer lines was impressive. All tests run at the factory were repeated after installation in the Nanofabrication Center, in order to verify the installation as well as isolate any site-specific sources of noise or interference (below). Test Min. Specification Factory Acceptance Site Acceptance Scan field distortion <15 nm 6 nm 4 nm Beam Current Stability <0.5% 0.131% 0.111% Field Stitching Error <20 nm 9 nm 13 nm Alignment Overlay Error <20 nm 10 nm 12 nm Min. Linewidth <8 nm 7 nm 6 nm Linewidth Variation <20% 14% 12% Table I: Major factory- and site-acceptance test results, as compared to the tool’s advertised minimum specification. Currently, there are about 45 active users on the system. The EBPG has proven itself to be an incredibly versatile tool since its installation. Its patterning resolution is typically high, as would be expected for an EBL tool in its class. While the 6-nanometer lines patterned during acceptance were not practical as anything but a tool demonstration, 10-20 nm resolution is routinely achieved for usable geometries and resist stacks. The text in the image provided is made up of features 15 to 20 nm wide, and demonstrates both geometric robustness and high resolution. The fast pixel clock and extremely high beam currents achievable on the tool have allowed it to be used for processes that are typically not practical to realize with EBL. The system can easily switch beam currents from 100 pA to almost 400 nA, three orders of magnitude. The beam diameter increases with beam current, but the ability to switch between such a wide variety of currents allows users to pattern nanoscale features using a fine beam, then use a high beam current to write coarse features without removing the sample from the system. The high maximum beam current on the tool has also allowed us to use it for mask fabrication. A typical 5" mask takes 8-12 hours to write on the tool, fast enough to be done overnight. The system has been very reliable, with system uptimes above 95% during the first year of installation. The two major downtime-causing issues encountered during this period were a firing unit failure (requiring a replacement unit) and the failure of a temperature-control bath during a loss of climate control in the lab. Both of these were fairly routine repairs and we were able to fix them quickly, although some unpredictable system behavior just prior to the firing-unit failure caused issues for some users during May-June 2011. In early May of 2012 an issue with the high voltage supply led to several weeks of downtime. The problem has been identified and corrected and the system is back in use. One of the major challenges with using the EPBG has been process development. To be able to optimize the process we acquired a refurbished Amray field emission electron microscope. It can resolve patterns created by the e-beam. We used the remaining funds from the MRI award, about 3% of the total, to fund it. Additional funds came from our National Nano Infrastructure Network award and a fund provided by the Dean’s Office to supplement the capital equipment acquisition. The EBPG has become an integral tool for nanoscale processing at the University of Minnesota, and is being used for a wide variety of electronic, material, biological, and other applications. The EBPG5000 is a world-class electron-beam lithography system, and should continue to draw in all types of users from academia and industry.

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University of Minnesota Twin Cities
United States
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