Credit: © 2006 ACS

The difficulty with patterning insulating substrates with electron-beam lithography (EBL) is the effect of charge buildup. Charging produces a large electric field at the surface, which can deflect the electron beam and result in pattern error.

To avoid this problem, Joseph Jacobson’s group1 at the MIT Center for Bits and Atoms have applied a relatively simple trick borrowed from silicon processing. At the electron acceleration voltages typically used for EBL, negative surface charge builds up from the electrons scattered within the insulating film. However, as the voltage is reduced, some of the electrons are backscattered away from the surface, leaving behind a net positive charge. The idea is to find the critical voltage that balances positive and negative charging and reduces the net electric field at the surface.

The critical voltage was determined by writing a series of broad area patches with different voltages on a 65-nm-thick film of poly(methyl methacrylate). Scanning electron microscope images of the patches showed that a minimum field was achieved with an accelerating voltage of 1.3 keV. A pattern with sub-100-nm features written at the critical voltage was then found to be true to the desired pattern, whereas the same pattern written at 5 keV had substantial defects. Jacobson and co-workers suggest that their technique will be useful for electron-beam writing in applications based on insulators, such as optoelectronics and flexible polymer-based electronics.