Improvements of Secondary Electron Imaging and Endpoint Detection in Focused Ion Beam Circuit Modification

Abstract

Abstract Secondary electron signal is widely used in Focused Ion Beam (FIB) systems for imaging and endpointing. In the application of integrated circuit modification, technology has progressed towards smaller dimensions and higher aspect ratios. Therefore, FIB based circuit modification processes require the use of primary ion beam currents below 10 pA and Gas Assisted Etching (GAE). At low beam currents, short pixel dwell times and high aspect ratios, the level of available secondary electrons for detection has declined significantly. FIB GAE and deposition requires delivery and release of a gaseous agent near the beam scanning area, and involves insertion of a gas delivery nozzle made of conductive material and grounded for charge prevention purposes. The proximity of a grounded gas delivery nozzle to the area being milled and/or imaged creates a “shielding” effect, further lowering secondary electron signal level. The application of a small positive bias to the gas delivery nozzle provides an effective way of reducing the “shielding” effect. Depending on the geometrical arrangement of the gas delivery system and other conductive objects in the chamber, an optimized nozzle bias potential can create conditions favorable for enhanced extraction and collection of secondary electrons. The level of the secondary electron image signal, collected in an FEI Vectra 986+ system, from a grounded copper sample with the nozzle extended and biased can be enhanced as much as six times as compared to the grounded nozzle. Secondary electron intensity endpoint is improved on backside samples, however shielding of the nozzle field by the bulk silicon substrate limits the electron extraction effect from within a via. For front side edits the improvement of endpoint signal level can be dramatic. Lateral image offset induced by the electrostatic field of a biased nozzle, can be removed by software position compensation.