Photolithography (Optical Lithography) Explained
Definition: Photolithography, also known as optical lithography, is a micro- and nanofabrication process widely used in semiconductor manufacturing and various nanotechnologies to create intricate patterns on a substrate, typically a silicon wafer. This technique employs light to transfer a geometric pattern from a photomask to a light-sensitive chemical photoresist on the substrate. Following exposure, the substrate undergoes a series of chemical treatments to develop the exposed pattern, which can then be used for etching or adding new material layers, forming the basis of integrated circuits and various microsystems.
How It Works
The process starts with the substrate being coated with a light-sensitive photoresist. A photomask, which contains the desired pattern, is then aligned above the substrate. Ultraviolet (UV) light is shone through the photomask, exposing the photoresist underneath except where the mask blocks the light. The exposed areas of the photoresist become soluble (in positive photoresist) or insoluble (in negative photoresist) to the developer solution. The substrate is then developed, washing away the soluble areas and leaving the patterned photoresist behind. This pattern can then guide further processes, such as etching or doping, to create the final microstructures.
Photolithography: Step by step
Applications
- Integrated Circuits (ICs): The primary application of photolithography is in the fabrication of integrated circuits. It enables the creation of complex circuit patterns on silicon wafers, which are essential for producing the myriad of electronic devices we use today, from smartphones to computers and beyond.
- MEMS and NEMS: Micro-Electro-Mechanical Systems (MEMS) and Nano-Electro-Mechanical Systems (NEMS) are made using photolithography to create tiny mechanical devices such as sensors, actuators, and microfluidic devices, which have applications in industries like healthcare, automotive, and consumer electronics.
- Photonics: In the field of photonics, photolithography is used to create optical components such as waveguides, gratings, and photonic crystals that manipulate light. These components are crucial for telecommunications, information processing, and sensing technologies.
- Biochips: Photolithography is employed in the manufacture of biochips, which are used in medical diagnostics, drug discovery, and biological research for the detection and analysis of biological molecules.
- Flat Panel Displays: This technology is also instrumental in the production of flat panel displays, including those used in TVs, computer monitors, and smartphones, facilitating the formation of pixels and color filters.
Advantages and Limitations
Photolithography offers high precision, repeatability, and the ability to fabricate complex micro and nanostructures across large areas. However, it faces limitations such as diffraction and scattering of light, which set bounds on the minimum feature size that can be achieved. Advanced techniques like extreme ultraviolet (EUV) lithography are being developed to overcome these limitations and reduce feature sizes further.
Further Reading
For an in-depth look at the various types on lithography being used in nanofabrication and nanoengineering, read our explainer: The Different Types of Lithography in Nanotechnology.