What is spin coating?

Spin coating is a method of depositing a thin film onto a substrate, such as a silicon wafer. At first glance, it is not a high-tech method; a wafer spins at high speed, and a liquid is dripped onto the substrate from above. The liquid strikes the substrate and is slung outward due to centrifugal forces. With the right combination of rotational speed, surface and liquid properties, a thin, relatively uniform coating is possible.

A wafer is held in place by a rotating chuck, which then spins at a set rotational speed. The chuck will either have a physical clamp or hold the wafer in place by pulling a vacuum against the backside of the wafer. Once the wafer is up to speed, the liquid is dripped at a controlled flow rate onto the surface of the wafer. The spread of the droplets on the surface depends on the surface roughness and wettability, as well as the liquid properties, such as the surface tension.

Spin coating applications

The semiconductor industry uses more spin coatings than virtually anyone else. During the numerous steps a silicon wafer undergoes to transform sand into a computer chip, there are many lithography steps. Each of these steps relies on a thin layer of photoresist, a chemical that bonds to the wafer or oxide layer and can be selectively exposed and removed. To ensure a thin, and thus predictable layer of photoresist, spin coating is the method of choice. This article will mostly discuss spin coating’s application in the semiconductor industry.

Spin coating controls

Engineers have several controls that they can adjust to optimize spin coated thin films. The most obvious control is the rotational speed of the chuck. If all other parameters are left constant, the faster the spin, the thinner the coating, as the centrifugal forces are stronger and overcome surface tension and van Der Waals forces more easily. However, if the wafer is spun too quickly, it can lead to uniformity by streaking and even wafer breakage.

The duration and flow rate of the liquid is another important consideration. Ideally, the perfect amount of material is deposited, and very little is slung off the sides of the wafer. If the total liquid is too small, the coating will not be complete. If too much liquid is used, the process can be wasteful and inefficient, as most of the common coating materials, such as photoresist, cannot be recycled. Furthermore, the dripping and spinning process quickly dries out the liquid by design. Therefore, if the liquid flow rate is too slow, there will be streaks on the wafer that dry and may not bond with other droplets. This non-uniformity will wreak havoc on product yield. However, if the liquid flow rate is too large, the coating can be too thick, piling up and drying slowly in the center. This is also a non-uniformity that will cause issues.

Other considerations

Depending on the application, oxygen-sensitive compounds can be spin coated under vacuum or an inert atmosphere. Certain photoresist compositions are deposited under these conditions. At the opposite end of the spectrum, some coatings can be deposited under reactive environments. These coatings are either applied first and then air, oxygen or another atmosphere released into the chamber to cause the reaction. While rare, some applications require this type of spin coating.

Sometimes, the rotational speed of the coating is adjusted during the spin. This is particularly true when excess water must be driven from a coating. In these applications, the coating is deposited, and as it begins to dry, the liquid flow is stopped, and the rotational speed is increased. This is a way to dry the wafer before the next processing step.