ALD involves alternating exposure of a substrate to different precursor gases. Each precursor reacts with the substrate surface, leaving a monolayer of material. A typical ALD cycle consists of four steps: 1. Exposure to the first precursor. 2. Purge to remove excess precursor and byproducts. 3. Exposure to the second precursor. 4. Purge to clean the chamber of any remaining gases. These steps are repeated until the desired film thickness is achieved.