Supercritical drying for high aspect-ratio HSQ nano-structures
Introduction
Electron beam lithography (EBL) is the exploratory lithography in the field of nanotechnology with the potential of ultimate resolution. To tap the full potential of an electron beam writer full control over the whole lithographic process, consisting of suitable resist material, exposure, development and resist drying, is indispensable. Hydrogen silses quioxane (HSQ) has been shown to exhibit particular attractive properties. In this high resolution negative tone inorganic electron beam resist fabrication of 20 nm wide 1:1 lines and spaces have been shown, using thin HSQ layers [1]. Single lines with even sub 10 nm line width have been realized and high contrast values have been achieved with optimized development processes [2], [3]. The small size of the molecules is mandatory for successful attempts to reduce line width fluctuations to a minimum [4]. The present work is focused on the drying process of dense HSQ patterns. Especially for thick HSQ layers, relevant as etch mask, two major problems have been found to occur when resist drying is performed by conventional nitrogen blow: (i) the surface tension of the liquid trapped between two adjacent structures causes pattern collapse of dense structures and (ii) liquid that effuse out of the resist matrix sweeps out polymer aggregates, giving rise to a certain degree of sidewall roughness, destabilizing fine resist structures and finally leading to the collapse of the structures. In order to overcome these two obstacles, a supercritical fluid with a significantly reduced surface tension and an increased diffusivity is used [5].
The results of supercritical resist drying (SRD) processes are compared with conventional resist drying by nitrogen blow to explore the potentials for the fabrication of dense structures in 220 nm high resist patterns. This HSQ resist thickness is suitable for transfer of well-defined patterns used for fabrication of functional devices for nanoelectronic, optoelectronic and life sciences [6].
Section snippets
Experimental
Silicon samples have been spin coated with HSQ to a thickness of 220 nm and baked on a hot plate for two minutes at 120 °C. For certain investigations the resist thickness has been increased up to 790 nm. Test patterns of double lines with varying line width and line spacing have been exposed with electron beam lithography using a Leica EBPG-5000 system operating at 100 kV.
Development has been carried out at 21 °C in high concentrated tetra-methyl ammonium hydroxide (TMAH). The development process
Results and Discussion
First, twin lines with a height of H = 220 nm and a line spacing of d = 50 nm have been investigated. At this line spacing a minimum linewidth of W = 40 nm corresponding to an AR of 5.5 is necessary to achieve stable twin lines for nitrogen blow drying. For narrower lines, such as shown exemplary in Fig. 1 for W = 33 nm, the mechanical stability of the lines is to low to sustain the force induced by the surface tension of the rinsing liquid. Consequently, pattern collapse is observed.
In contrast, clearly
Conclusion
The potential of SRD processes for high AR and dense HSQ patterns using CO2 as supercritical fluid has been demonstrated. For 220 nm high SRD dried resist structures a maximum AR of 17 has been observed. This value is up to three times higher compared to nitrogen blow dried resist structures. Further, improvements for widely spread structures have been shown. These results demonstrate the decisive benefits of supercritical resist drying processes for maximum achievable aspect ratios and the
Acknowledgement
This work has been supported partially by the Ministry of Innovation, Science, Research and Technology of the State Northrhine-Westfalia, Germany.
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