The deposition is often based on chemical reactions, and the more challenging the process the more precise control over the chemistry is required. Chemical vapor deposition (CVD) is the workhorse for depositing thin films of materials and has been used since the early days of chipmaking. CVD is based on chemical reactions between volatile precursor molecules, containing the atoms needed to form the film material, and the surface where the film is depositing. While CVD is mainly known, and used, for depositing films over large areas of flat surfaces, high precision methods have been developed for filling more complex architecture, such as holes or trenches. The technology that excels at precision is the form of CVD known as atomic layer deposition (ALD). ALD is a time-, or spatially resolved form of CVD where the precursors for the metal and non-metal atoms are introduced to the reactor separately. This means their only chance for reacting is with surface moieties, formed by surface reactions with the previous precursor and the surface. This builds a material, ideally, atomic layer by atomic layer with great precision. ALD was first patented 1974 and has been crucial to the semiconductor industry since the early 2000's when ALD proved to be the enabler for the high-k transistors, where HfO2 deposited by ALD replaced thermally grown SiO2 as the gate oxide in silicon-based transistors. ALD is now used to deposit a wide range of materials, for example oxides, nitrides, sulfides, metals, and even organic and polymer films.