Investigation of Ga contamination due to analysis by dual beam FIB

Ga contamination in wafers analyzed by a dual beam FIB was investigated. For a milled wafer the following has been found. Surface sensitive methods, TRXRF and TRXPS, indicated that the Ga contamination level is of the order of 10¹⁰ atoms/cm² and is insensitive to injected dosage. ICP-MS analysis, which detects species in surface layers of about 5 μm thick over a whole wafer detected Ga of 1.1×10¹³ atoms/wafer against a 71 min.nA dose and 5.1×10¹³ atoms/wafer against a 567 dose. These are related to dosage. Most of the irradiated Ga will charge up in the bottom of milling crater and only a very small fraction of them will contaminate the surface area. The WDX analysis indicated that Ga distributes mostly within the area of 200 μm φ centered on the milled crater The AES analysis also showed similar results. The lateral Ga distribution was surveyed by TRYRF for a total of 49 spots of 10 mm φ, including the milled spot and its neighboring spots. Ga was detected in 2 spots. Taking it into account, we should regard that the area within 200 mm φ is contaminated and the rest of the surface is clean. A further investigation is necessary in order to determine whether a milled wafer can be returned to a production line or not. For a wafer tested without ion milling, no Ga was detected by either ICP-MS or TRXPS. The wafer tested without ion milling can be returned to a production line