Semiconductor quantum dots (QDs) are electronically-quantized systems with promising applications in optoelectronic devices ^[1] . A key aspect of such systems is the fine control of optical transitions in the synthesis process ^[2] . These QDs are predominantly used in thin-film arrangement, deposited by spin casting or dip coating. Single QD patterning is one of the major challenges to designing a system that takes advantage of individual properties of QDs. Here we present a template self-assembly technique to control the position of individual QDs through electron-beam lithography (EBL). This optimized top-down lithographic process is a step towards the integration of individual QDs in optoelectronics systems for industrial applications.

The fabrication process of templated QDs is illustrated in Figure 1a. A poly(methylmethacrylate) (PMMA) resist was spin coated on a silicon substrate, followed by the fabrication of a mask through EBL. The size of the resulted PMMA templates was minimized by varying development temperature ^{[3] [4]} . Figure 1b shows the optimized PMMA patterning, with minimum template (i.e., hole) size of 8 nm for development at 6 °C. After defining the PMMA templates, a solution of QDs (6-nm-diameter CdSe) was spin casted and the remaining resist was removed by dissolution in acetone. This process resulted in QD clusters attached on the substrate. By optimizing the QD solution concentration, resist thickness, and feature size, we fabricated clusters with 1 to 10 QDs. One figure of merit in this process is the pattern yield, which is the ratio of yielded structures to the patterned templates. Figure 2 shows QD clusters with 87% pattern yield, with an average of 3 QDs in each cluster. Control of QD placement will be further optimized and integrated into photonic devices.

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4. B. Cord, J. Lutkenhaus, and K. K. Berggren, “Optimal temperature for development of poly(methylmethacrylate),” J. Vac. Sci. Technol. B vol. 25, pp. 2013-2016, Dec. 2007.

A commercially-available scanning-helium-ion microscope of high source brightness ^[1] has been modified for operation with neon gas. This neon system had been evaluated for nano-machining ^[2] , but not for resist-based lithography, as has been done with helium systems ^{[3] [4]} . The neon system may enable a lithography process with higher resolution than any scanning-particle system to date. This possibility is due to the combination of the high-brightness source and the expected reduction of secondary-electron (SE) range relative to electrons or helium ions. In addition, the expected increase in SE yield relative to electrons or helium ions may lead to a lithography process with high sensitivity. This high sensitivity could allow critical doses below substrate-damage thresholds. Figure 1 presents preliminary data on the point-spread function (PSF) of neon compared to helium.

The Stopping and Range of Ions in Matter (SRIM) is a popular, industry-standard tool for simulating the trajectories of incident ions in a target sample. However, SRIM does not simulate the trajectories of secondary electrons (SEs) produced by ion-sample interactions. SEs are responsible for exposure of resist and thus figure prominently in modeling of electron-beam lithography and proton-beam lithography. We developed a hybrid approach to modeling helium-ion lithography that combines the power and ease-of-use of SRIM with the results of recent work simulating SE yield in helium-ion microscopy ^[5] . This approach traces along SRIM-produced helium-ion trajectories, generating and simulating trajectories for these SEs using a Monte Carlo method. Figure 2 illustrates the utility of our software, which can also simulate electron beams.

1. B. W. Ward, J. A. Notte, and N. P. Economou, “Helium ion microscope: a new tool for nanoscale microscopy and metrology,” J. Vac. Sci. and Technol. B, vol. 24, pp. 2871-2874, 2006.
2. S. Tan, R. Livengood, D. Shima, J. Notte, and S. McVey, “Gas field ion source and liquid metal ion source charged particle material interaction study for semiconductor nanomachining applications,” J. Vac. Sci. and Technol. B, vol. 28, pp. C6F15-C6F21, 2010.
3. D. Winston, B. M. Cord, B. Ming, D. C. Bell, W. F. DiNatale, L. A. Stern, A. E. Vladar, M. T. Postek, M. K. Mondol, J. K. W. Yang, and K. K. Berggren, “Scanning-helium-ion-beam lithography with hydrogen silsesquioxane resist,” J. Vac. Sci. Technol. B, vol. 27, pp. 2702-2706, 2009.
4. V. Sidorkin, E. van Veldhoven, E. van der Drift, P. Alkemade, H. Salemink, and D. Maas, “Sub-10-nm nanolithography with a scanning helium beam,” J. Vac. Sci. Technol. B, vol. 27, pp. L18-L20, 2009.
5. D. Winston, J. Ferrera, L. Battistella, A. E. Vladar, and K. K. Berggren, “Modeling the point-spread function in helium-ion lithography,” submitted for publication.