C 2015 American Chemical Society Area-Selective Atomic Layer Deposition: Conformal Coating, Subnanometer Thickness Control,

I n the past few decades, semiconductortechnology improvements have beenextensively driven by electronic device scaling. Since the high-κ/metal gate device configuration was adopted for the Si tran-sistor design by Intel in 2007, atomic layer deposition (ALD) has become one of the enabling fabrication techniques to push device performance and energy efficiency forward.1,2 Currently, transistors are depart-ing from conventional planar structures and are moving into three dimensions with the current flowing on three sides of the device channel (i.e., top, left, and right in the 22 nm trigate FinFET technology); the advances of ALD achieving pinhole-free high-κ films over 3D channels has been critical for this development.3 Unlike the conventional gate dielectric of SiO2, which can be grown on Si by thermal oxidization, ALD is a deposition scheme that enables the con-formal coating of dielectric films with sub-nanometer control in thickness from vapor sources.4,5 In contrast to standard physical and/or chemical vapor depositions, an ALD process is implemented by alternating pulses of gaseous chemical precursors that react with the substrate.5 During each ex-posure, surface reactions occur only at the reactive sites, which restrict the formation of films to one atomic layer at a time. This self-limiting nature of surface reactions minimizes the influences of any uncontrol-lable parameters (e.g., randomness of the precursor flux) during film coating because extra precursors are removed and evacu-ated after each pulse. As a result, the de-posited ALD films are atomically smooth and conformal to the substrate, even with high aspect ratio nanostructures and, thus, * Address correspondence t