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.
This laboratory-scale spin coater shows a glass slide that is spun on a vacuum chuck. The chamber is lined with aluminum foil for ease of cleaning between test runs.
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.
In many cases, the liquid consists of a solvent and dissolved particles. These particles will either be deposited as the solvent reacts, or will polymerize when exposed to air, or may even react chemically in another step, such as heating. Some spin coaters can be heated and cooled to change the viscosity and surface tension of the liquid, and to start or prevent certain reactions.
Thin films
The thin films are mechanically bonded and chemically bonded to the surface of the wafer in most cases. The mechanical bonds depend heavily on the surface roughness. For silicon wafers in semiconductor processing, there is little mechanical bonding. For textured surfaces, the amount of mechanical bonding increases.
The surface roughness of the material can be checked with a profilometer, which traces along the surface with a needle and cantilever, measuring the tiny motions of the needle. However, most semiconductors are very smooth and the coating materials are designed to form a chemical bond instead.
Chemical bonding is often of the van der Waals variety. The van der Waals bonds are an attractive force that sticks two materials together because of the uneven distribution of charge in each molecule. At first glance, this seems like a weak force, but it has many useful applications, such as holding paint to virtually anything.
To take advantage of the van der Waals bonds, the substrate and the coating must be in very close contact. This means the wettability of the substrate is also important. Imagine the scenario where a piece of wax paper is to be covered with a thin layer of water — it simply won’t happen! The wettability of the liquid on the substrate can be checked by measuring the contact angle when a single droplet is placed on the surface of the substrate. If the droplet beads up, the liquid must be modified with a surface tension reducing compound. If the liquid spreads out, it will be suitable for use in the coating.
The other important feature of the liquid is its surface tension. While it can affect the contact angle, it has a much larger role in the overall shape of the coating. Molasses may wet the substrate, but its high surface tension means it would need to be spun at a higher rotational speed to properly coat it.
The surface tension also plays a role in the “sombrero effect,” where the coating tends to be the thickest at the center of the substrate, where liquid is typically dripped, and near the outer rim, where the liquid bunches up before being slung over the edge. The profile of the coating can look like a large sombrero, where the center and the outer rim are higher than the middle. Coatings with low surface tension reduce the sombrero effect, but it is always present. Semiconductor engineers work around this by building the effect into their designs.
Final thoughts
At first glance, spin coating is a low-tech solution for creating thin coatings. Upon further inspection, there is plenty of fluid mechanics, chemistry and design that goes into a properly developed spin coating. It is a workhorse for the semiconductor industry, used to spin thin films of photoresist onto the wafers before exposure during the other lithography steps.
