What is it?

Photolithography is the process used to apply patterns onto a silicon wafer. This process is typically repeated several times, until multiple layers (20+) are created. These layers can consist of different materials, and all of them are interconnected (wired) very specifically. All of them can also be doped.

This process is done in order to make silicon wafers useful in computing, since they don't provide any function by themselves. Silicon is a renowned semiconductor, chosen for its ability to conduct or not conduct current, which is a useful property for producing electronic circuits. This property is useful because computers operate in binary at the lowest level; in 0s and 1s. Those 0s and 1s are simulated physically by semiconductors, like silicon, by alternating between conducting and not conducting current.

Photolithography is an especially important factor in improving the performance, energy efficiency, and density aspects of semiconductors. This is because the size of lasers used during this process determine the size and form of the structures formed out of silicon wafers. These structures such as transistors, resistors, diodes, and capacitors.

The size of the lasers used for forming these structures is a characteristic of what's known as a "process node". These process nodes are named accordingly to their primary characteristics (e.g. laser size), so as to help differentiate different lithography processes from each other. There are several semiconductor fabrication firms around the world that develop at their own pace and with their own characterstics from one process node to another, so being able to differentiate them all by names and specifications is crucial.

As process nodes have improved since the inception of integrated circuit processors, the size of their components (mostly transistors) have shrunk from thousands and hundreds of nanometers to only several tens of nanometers today. This means there is greater density of these individual components in these ICs, as well as improved performance and/or energy usage. It's important to note that only 2 of these 3 improvements can be maximised at once, so engineers have to decide what characteristics they wish to improve with each successive process node.

One other significant aspect of photolithography is the economics of the process. It's not an inexpensive process to carry out, let alone carry out well; consequently, there are only a handful of major semiconductor fabrication firms around the world. Each of these "fabs" has to maintain good financial standing in order to not collapse under debt. The economics of each process node is heavily determine by the yields of each silicon wafer. This is simply how many fabricated ICs on a wafer perform as intended out of the total number of ICs physically on the wafer. This is usually expressed as a percentage (e.g. 90% yields for a 14nm process node). Additionally, the way the photolithography process is carried out, there are more functional ICs towards the center of a wafer than around the edges of the wafer, so this is always accounted for before a wafer is processed.