Pixel spacing effects for nanofabrication using focused ion beam

Abstract

Focused ion beam (FIB) systems are widely used as a versatile tool for nanofabrication prototyping, device modification and ion beam lithography. However, there are still many unexplored effects due to the different methods that ion implantation could perform during FIB milling using Ga as a liquid metal ion source. In this report we studied the effects of pixel spacing when FIB is used for direct milling of a substrate at different milling currents for constant implantation doses. The experiment consists of FIB milling of a Si substrate at 30 keV using currents of 50 pA and 100 pA for a dose of 5×1016 ions/cm2. The dwell time was set to be 1 μs and the pixel spacing varied from 6.2 nm to 34.2 nm. The surface topographies of machined regions were examined using the atomic force microscope and the quality is described by comparing the intensities of a crystal to amorphous peak of the recorded trace from Raman spectroscopy measurements. This method was introduced by Wagner. In order to more accurately consider the sputtering yield the effect of second order deposition was neglected. It was observed that by increasing the pixel spacing the sputtering yield starts to increase and then gradually decreases for both currents. Ion implantation breaks the crystal structure and the process involves displacement of the atoms from the atomic rows which consequently increases the effect of de-channeling in ion implantation. The increase in sputtering yield could be because of the enhanced de-channeling which is due to the changes of the substrate structure which increases the collisions between implanted ions and the substrate atoms. After a threshold, the sputtering yield gradually decreases, which is due to having less implanted ions per unit of volume for each scan and therefore less applied damage. Pixel spacing at different currents and dose rates can yield different behavior due to the concentration of implanted ions per pixel dwell time. In our study the maximum concentration of implanted ions per pixel dwell time is about 5×1017 ions/cm3 and 1018 ions/cm3 for currents of 50 pA and 100 pA respectively; this concentration is lower than 1019 ions/cm3 which is the saturation point of Ga solubility in Si. It was observed that increasing the pixel spacing leads to rougher surfaces. It was also found that the quality of Si is at its highest when the pixel spacing is 14.8 nm. This is consistent with the topography results which were described by the de-channeling effect. As the de-channeling increases, the depth of implanted ions is decreased, and therefore fewer layers of substrate are damaged. In this study, we investigated the effect of FIB milling pixel spacing on substrate physical and structural changes at a dose of 5×1016 ions/cm2. We observed the sputtering yield is first increased and then decreased, which is mainly due to structural changes in substrate. The quality of substrate was also studied, revealing less damage when the pixel spacing is 14.8 nm for both currents