[MUSIC] Hi, I'm Steve Shannon, Associate Professor in the Department of Nuclear Engineering at NC State University. Today, we'll be talking about etching. Etching is a method used in nanofabrication to remove unwanted material. For example, let's say you have a substrate made of silicon that has been coated with a thin film of silicon dioxide. Right now, the silicon dioxide completely covers the surface. However, if you want it in a specific shape, like a checkerboard pattern, you need to protect the silicon dioxide you want to keep and leave the silicon dioxide exposed for chemical removal that you want to remove. This could be done using a photolithography process. In photolithography, light sensitive polymeric photoresists are deposited on the top of the thin film surface. They are then cured in precise patterns using light. After creating this type of mask, the substrate is exposed to an etching agent, or etchant, that will attack and remove unwanted silicon dioxide. The mask prevents the underlying material from removal by the etchant. After this process is complete, the desired checkerboard pattern of silicon dioxide remains. The masking agent can later be removed through different processes depending on the material and how it was deposited. There are some common terms when discussing the etching process, including etch rate, selectivity, and directionality. The etch rate is the rate of material removal. If you are etching a thin film, it is important to know how quickly the film will be removed. This will help you determine how long to expose the substrate to the etching. It is also critical to know the etch rate of any other materials in the system, especially those you don't want to remove. These etch rates can be altered by changing parameters of the reaction, such as temperature and pressure. Etch rates are known for certain material and etching combinations. However, as many factors can affect the etch rate, it is critical to determine an etch rate with a test substrate for your own process design. Another important parameter is selectivity. Etch selectivity is the ratio of etch rates of different materials in the system. These materials include the substrate, the deposited film or films, and the protective materials, such as the mask. When etch rates of the mask and the underlying substrate are close to zero or low when compared to the etch rate of the film, then we say that the etch selectivity of the film is high. This is normally desired. If the etch rate of the mask or the substrate is close or equal to the etch rate of the film, then the selectivity is low. This etch selectivity, particularly selectivity to the mask material and underlying layer, will determine how deep into the etch material you can go and also dictate how uniform your etch process needs to be over the entire wafer. We want to avoid complete removal of our mask layer and any removal or damage to the material underneath the etch layer that we want to keep. Finally, the direction of the reaction is important. In isotropic reactions, the etching will remove material in all directions equally. In other words, the etch rate is the same in all directions. This is typically seen in wet etch processes. This is problematic if material underneath the productive layer is removed, as seen here. This is termed undercutting. Anisotropic is the rate at which we etch vertically into a material divided by the rate at which we remove material underneath the mask. For ideal pattern transfer, the side walls are vertical and perfectly aligned to the edge of the mask. This is an example of infinite anisotropic. In some cases, we do not want a perfectly anisotropic process. For example, etch trim is used to achieve features that are smaller than can be achieved through photolithography. This is an example where infinite anisotropy is not ideal. This control of etch rate, as a function of direction, helps features maintain the desired size and shape. This concludes our overview of etching. Thank you for joining us today.