Limiting factors for electron beam lithography when using ultra-thin hydrogen silsesquioxane layers

Abstract

Abstract. Isolated dots and lines with 6 nm width are written in 20-nm-thick hydrogen silsesquioxane ͑HSQ͒ layers on silicon substrates, using 100-keV electron beam lithography. The main factors that might limit the resolution, i.e., beam size, writing strategy, resist material, electron dose, and development process, are discussed. We demonstrate that, by adjusting the development process, a very high resolution can be obtained. We report the achievement of 7 nm lines at a 20-nm pitch written in a 10-nm-thick HSQ layer, using a potassium-hydroxide ͑KOH͒-based developer instead of a classical tetra-methyl-ammonium hydroxide ͑TMAH͒ developer. This is the smallest pitch achieved to date using HSQ resist. We think that the resolution can be improved further, and is presently limited by either the beam diameter ͑which was not measured separately͒ or by the not-fully-optimized development process.

Resists for sub-20-nm electron beam lithography with a focus on HSQ: state of the art

“…The main risk of using a strong developer when writing nanostructures is that the structures may be washed away and only highly exposed areas remain on the substrate. This effect has been suggested by Grigorescu et al [72] when writing arrays of 6 nm dots in a 20 nm HSQ resist layer using two different concentrations of TMAH developer. MF-322 was used undiluted (N = 0.268 eq l −1 ) and diluted with demineralized water in a ratio 1:3 (N = 0.067 eq l −1 ).…”

“…Grigorescu et al [72] reported a recipe to obtain ultrathin HSQ resist layers. By using a higher dilution rate, 1:10 FOx-12:MIBK, a 10 nm thick HSQ layer was obtained at 3000 rpm for 60 s on a Karl Suss spinner with the lid closed (in order to decrease the rate of evaporation of the solvent) (see figure 20).…”

“…There is little information regarding the writing strategy of small structures in HSQ. Grigorescu et al [72] suggested that when small features are desired, the ultimate resolution is set by various factors such as beam size, resist material, exposure dose, development process and also the writing strategy. Several writing strategies were discussed and an ideal recipe for obtaining small structures was presented.…”

“…By doing a dose test, an optimum dose can be determined for which the linewidth after exposure and development is equal to the designed linewidth. By using n-exel single pass exposures, sub-10-nm semi-dense lines were written in a 10 nm HSQ resist layer [72] (see figure 23).…”

“…The development time is usually fixed and the optimization of the development process is done by varying other parameters, e.g. baking temperature, concentration of the developer [72,73]. When nanostructures are written in ultrathin resist layers, the development time might become an important resolution-limiting factor.…”