[MUSIC] Hello. My name is Veena Misra. I'm the Director of the NSF Nanosystems Engineering Research Center, called ASSIST, which stands for Advanced Self-Powered Systems of Integrated Sensors and Technologies. I am also professor in the Department of Electrical and Computer Engineering at NC State. Today I will show you the basics of a nanofabrication technique called atomic layer deposition or ALD. ALD is a subclass of a larger family of deposition processes called chemical vapor deposition or CVD. To understand ALD, it is important to know about CVD processes in general. CVD techniques are used to deposit thin layers of films of materials at the nanoscale onto a base material. The process involves exposing a base material to gas-based sources that react and deposit a film on the surface. This base material is also call a substrate, a foundation on which the film will grow. The substrate is exposed to multiple gases which react with each other and cause a thin film to grow on the surface of the substrate. The thin films have unique properties that add new functionality to the substrate. We add these films to improve the original material by adding new properties. Some of these functionalities include electrical properties, like conductivity for electronic components, or making a material have better barrier properties, like making it waterproof. In most CVD processes, the substrate is placed in a chamber and all of the precursors are introduced at the same time. The presence of the gases, along with heat or other stimuli, cause the film to grow onto the substrate. ALD is different from other CVD processes in that the precursors are sequentially introduced to the chamber to build one atomic layer at a time. The substrate is exposed to the first precursor, which reacts with the surface of the substrate and grows a single layer of molecules. Any unreacted first precursor is then purged out of the chamber. This is called a purge cycle. Then, the second precursor is introduced into the chamber. This builds another layer of molecules onto the last layer. Then, another purge cycle removes the second precursor from the chamber. These steps combined are called a cycle, that provide one monolayer of the film, typically one angstrom in thickness. These steps can be repeated as many times as desired to build the film layer by layer. Now let's take a look at how this takes place in an actual ALD system. Here, a silicon wafer is placed into the chamber, where we will grow an aluminum oxide film onto it using trimethylaluminum, TMA, and water as our precursors. This type of process is routinely used to make dynamic random access memory chips, also called DRAM chips. The chamber is pumped down and the first precursor, TMA, is released into the chamber. Once the TMA is out of the chamber, we let our second precursor, water vapor, flow in. By doing this, we are slowly adding molecules one at a time to the surface of the material. Now let's take a closer look at how this happens. The silicon wafer has hydroxyl groups on the surface that make great bonding regions for films to grow on. Hydroxyl groups consist of one oxygen atom, the yellow atoms, and one hydrogen atom, the blue atom. The TMA flows in, reacts with the hydroxy group on the surface, and leaves a layer of aluminum atoms onto the surface of the wafer, the purple atoms. When all of the available sites are occupied, no more TMA molecules can attach. The excess TMA and methyl groups must be pumped out with a purge cycle. Next, we flow our second precursor, water vapor, into the chamber. Here, the water vapor comes in and creates oxygen bridges between aluminum atoms on the surface and also adds a new layer of hydroxyl groups to attach more aluminum atoms to. We can repeat these steps as many times as we want until we have the desired amount of layers of aluminum oxide. In ALD, it is very easy to coat complex surface shapes like trenches and nanowire networks. ALD can be used to deposit many different films on a variety of materials for different uses. The sequential alternating nature and self-limiting reaction of ALD also makes it very easy to control the thickness of the layer that we deposit. We are essentially able to control how many atomic building blocks we build on to the original surface. ALD is reliant on reactive chemistry and the sequential nature is self-limiting. In other words, the reaction terminates when precursor molecules have no more sites available to attach to. This makes it easier to get a truly conformal film on geometrically complex material surfaces like fibers. ALD does not require high temperatures to allow the surface reactions to take place. This means we can deposit onto materials that might be destroyed or melt in high heat, such as polymers or biological samples. However, ALD also has disadvantages in comparison to other CVD processes. Because we are relying on surface reactions, this puts limitations on what type of materials we can deposit onto, and requires compatibility of precursors. ALD is also very slow in comparison to most CVD processes. Therefore, it is difficult to grow thick films more than 100 nanometres. This covers the basics of atomic layer deposition. Thank you for watching this video.
