Michael Dickey covered a number of interesting uses of liquid metals in his article “Liquid metals at room temperature” (Physics Today, April 2021, page 30), but he left out one that has had great practical importance: the liquid-metal ion source (LMIS). Developed in the 1970s,1 the LMIS revolutionized focused-ion-beam (FIB) technology by allowing usable currents (1–10 pA) to be focused to a diameter of only a few nanometers. Most FIB systems use liquid gallium because of its low vapor pressure at room temperature, its ability to stay liquid below the melting point, and its high surface tension.
Among other applications, LMIS-based FIB is used in semiconductor manufacturing. High-resolution FIB makes it vastly easier to do failure analysis on submicrometer-scale integrated circuits. The technology also enables the rewiring of integrated circuits in the development stage: Conductors can be cut and new ones added through the deposition of metallic compounds. That allows design engineers to make modifications to a circuit without needing to produce a new mask set each time. Hundreds of papers and at least three books2–4 have covered LMIS technology and its applications.
