Defects and gallium-contamination during focused ion beam micro machining

Low energy focused ion beam (FIB) systems are used for the modification of integrated circuits, the preparation of TEM samples, the processing of structures in the sub -100 nm range, and for failure analysis. Focused gallium ion beams with diameters down to <10 nm (Full Width at Half Maximum) allow high resolution secondary electron surface inspection as well as local removal and deposition of material with high accuracy by physical sputtering or ion induced chemistry. Besides beam diameter and shape, gallium implantation and defect generation limit the application of FIB micro machining towards minimum resolution and sensitive analysis. Even when the desired structural dimensions are achieved, functionality of the sample may be hindered by implanted gallium and introduced defects. In this paper, vertical gallium distribution and lateral contamination for highly focused ion beam applications are investigated. SIMS analysis and Monte Carlo simulation are used for the determination for doses ranging from FIB inspection (1·10¹⁴ cm^-2) to sputter removal at high doses (up to 1·10¹⁸ cm^-2). The measurements revealed surface concentrations of 1·10¹⁸ cm^-3 up to 6·10²¹ cm^-3 and deep ranging tails. At doses exceeding 1.10¹⁷ cm^-2, the surface concentration reaches 6·10²¹ cm^-3 and saturates, which is in agreement with simulations. Lateral contamination is determined by gallium background implantation due to inspection before processing and by non scanned neutrals. Defect generation was investigated by TEM. Even for typical FIB inspection at low doses (30 keV, 1·10¹⁴ cm^-2) an amorphous layer is generated with a thickness of 50 nm