We report the utilization of a focused mega-electron-volt (MeV) proton beam to write accurate high-aspect-ratio structures at sub-100 nm dimensions. Typically, a MeV proton beam is focused to a sub-100 nm spot size and scanned over a suitable resist material. When the proton beam interacts with matter it follows an almost straight path. The secondary electrons induced by the primary proton beam have low energy and therefore limited range, resulting in minimal proximity effects. These features enable smooth three-dimensional structures to be direct written into resist materials. Initial tests have shown this technique capable of writing high aspect ratio walls of 30 nm width with sub-3 nm edge smoothness.

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We report an alternative technique which utilizes fast-proton irradiation prior to electrochemical etching for three-dimensional microfabrication in bulk p-type silicon. The proton-induced damage increases the resistivity of the irradiated regions and acts as an etch stop for porous silicon formation. A raised structure of the scanned area is left behind after removal of the unirradiated regions with potassium hydroxide. By exposing the silicon to different proton energies, the implanted depth and hence structure height can be precisely varied. We demonstrate the versatility of this three-dimensional patterning process to create multilevel free-standing bridges in bulk silicon, as well as submicron pillars and high aspect-ratio nanotips.

With the rapid advances being made in novel high-energy ion-beam techniques such as proton beam writing, single-ion-event effects, ion-beam-radiation therapy, ion-induced fluorescence imaging, proton/ion microscopy, and ion-induced electron imaging, it is becoming increasingly important to understand the spatial energydeposition profiles of energetic ions as they penetrate matter. In this work we present the results of comprehensive yet straightforward event-by-event Monte Carlo calculations that simulate ion/electron propagation and secondary electron ͑␦ ray͒ generation to yield spatial energy-deposition data. These calculations combine SRIM/TRIM features, EEDL97 data and volume-plasmon-localization models with a modified version of one of the newer ␦ ray generation models, namely, the Hansen-Kocbach-Stolterfoht. The development of the computer code DEEP ͑deposition of energy due to electrons and protons͒ offers a unique means of studying the energy-deposition/redistribution problem while still retaining the important stochastic nature inherent in these processes which cannot be achieved with analytical modeling. As an example of an application of DEEP we present results that compare the energy-deposition profiles of primary MeV protons and primary keV electrons in polymethymethacrylate. Such data are important when comparing proximity effects in the direct write lithography processes of proton-beam writing and electron-beam writing. Our calculations demonstrate that protons are able to maintain highly compact spatial energy-deposition profiles compared with electrons.

We report a technique for fabricating enclosed nanochannels in poly(methylmethacrylate) (PMMA) using proton beam writing coupled with thermal bonding. Using proton beam writing, straight-walled high-aspect-ratio channels can be directly fabricated through a relatively thick PMMA resist layer spin coated on a Kapton film. By thermally bonding the fabricated structures onto bulk PMMA, peeling off the Kapton substrate, and bonding the remaining exposed side to PMMA, enclosed high-aspect-ratio nano/microchannels can be fabricated. Such enclosed channels can be incorporated into fluidic polymeric devices, and the process is compatible with the fabrication of multilevel three-dimensional fluidic chips with vertical interconnects.