Knowledge Base Fact Sheet
Introduction to the basics of photolithography
Lithography at its most general is the process of patterning the surface of a substrate to build up a complex design or structure.
Photo (UV) Lithography
Photo (or UV) lithography is the technique of pattering through the use of a light sensitive polymer (called photoresist) and a stencil (called a photomask)
The first step in any photolithographic process is to prepare the substrate – in most cases the substrate is a silicon wafer but can in theory be any material. Substrate preparation is undertaken to improve the adhesion of the photoresist to the substrate. Typical steps include substrate cleaning – to remove any dirt/contaminates, dehydration bake – to remove any water and then the addition of an adhesion promoter. Going through these steps will reduce the number of contaminates both organic and inorganic which will ensure the best results as we move through the lithography process.
Once the substrate has been prepared the photoresist can be applied to the surface. To get the best results a thin uniform coating is required; this thickness is controlled through a process called spin-coating. The photoresist is deposited onto the substrate. This is then spun on a turntable to 1000s of rpm spreading out the viscous photoresist into a thin layer. This thin resist is then soft-baked on a hotplate to remove excess solvent and to stabilise the resist film.
The next step in the photolithography process is to align the resist covered substrate to the photomask and to expose to UV light. The critical principle in photolithography is that the solubility of the photoresist is changed once exposed to UV light. Photoresists come in two categories: positive or negative.
In a positive resist, the part of the film that is exposed to the resist becomes more soluble and can be removed with a developer. In a negative the opposite happens, where the resist has been exposed to light the resist becomes harder and can’t be removed by the developer (see Figure 1d). There are many different compositions and versions of resist, allowing for different heights, temperatures, exposure settings and structures to be manufactured.
When aligning the substrate to the mask we use a mask aligner or a stepper to control where the pattern on the mask is projected onto the substrate below. A mask aligner is a faster process taking a pattern the same size as the wafer and projecting it onto the wafer. A stepper takes a small pattern and exposes that onto the wafer before moving the substrate a small way and exposing the same pattern onto a different part of the wafer – replicating the same patter over and over again.
The crucial aspect of the align and expose step is the exposure of the photoresist with UV light. The UV source can either be a traditional broadband mercury light or the more recent advance using a UV LED array. The UV source defines the resolution (smallest features) of the lithography process. A rough rule of thumb is that the resolution achievable is slightly more than half the wavelength of light. UV light has a wavelength of ~465nm, so the resolution achieved will be ~250 nm.
Once exposed the next step is to develop the resist. The developer will wash away either the exposed or unexposed parts of the resist film depending on the nature of the photoresist. Development is either done as a spray, where a fine mist of developer is sprayed onto the exposed substrate or as a puddle, where a pool of developer is poured onto the exposed substrate. In both cases it will remove the unwanted photoresist leaving the desired pattern.
Now the substrate is ready for the next step of the processing, either the deposition of a thin film onto the surface of the substrate or the etching and removal of the substrate. These techniques can be combined together with multiple iterations of photolithography to make complex designs and patterns.
25 October 2019
IKB038 Rev. 1