Knowledge Base Fact Sheet
What Is Spin Coating?
This document overviews the semiconductor fabrication process known as “spin-coating” and explains how it works (IKB-075).
What is spin coating?
Spin-coating is the most widely deployed method for dispensing photoresists and other materials uniformly onto substrates. Spin-coating is used to produce thin films of the desired material with high levels of process control and repeatability.
Photoresist as discussed in the lithography basics knowledge base document is crucial to the photolithography process. Typically, photoresist is a highly viscous material and the uniformity of its coating plays an important role in the reliability of any photolithography process, as well as the resolution achievable. Spin-coating is a technique widely used in research, development and industrial processes, in order to produce specific uniform film coatings.
The principle of spin-coating is that a few millilitres of photoresist are dispended onto the substrate. The substrate is then spun at high speeds in the range of 500 – 4000 rpm. The viscosity of the photoresist is then selected to keep the spin speed in the optimal range whilst producing a coating of the required film thickness. These parameters are usually specified by the resist manufacturer and are specific to the resist used. The other source of information for these parameters is to research the wide body of literature about photolithography and to adapt a published process to your needs.
The photoresist is dispensed at the centre of the substrate prior to the spinning, this is called static dispense. An alternative to this is dynamic dispense, where the wafer is already spinning at the desired speed and to then dispense the photoresist. This is the more commonly used technique when spin speeds are in excess of 1000 rpm.
At these high spin speeds the centrifugal force causes the viscous solution to spread outwards and flow towards the edge of the wafer. At the edge material builds up until the surface tension of the photoresist solution is overcome, at which point the resist is ejected from the spinning wafer. The thickness of this thin film is defined by a number of parameters: spin speed, concentration, viscosity and spin time.
The requirements for uniformity are demanding on the spinning process, as the quality of the film is critical to the number and size of defects in the pattern transferred. Photolithographic processes can require high uniformity, both across a single wafer and from one wafer in a cassette to another. With typical photoresist film thicknesses of 2 µm, this is a uniformity requirement of ±1.0%.
To achieve results with uniformity to this high tolerance the spin speed and time must be precisely controlled, as well as the acceleration of the wafer up to the specified spin speed. In some cases a multi-speed spin protocol is recommended by the resist manufacturer, this is to ensure that the final resist thickness is finely controlled and within the tolerances required for further processing.
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04 November 2020
IKB075 Rev. 1