Optical Lithography and Light Sources The semiconductor lithography process
starts with a light source and ends with the microchips that power your technology. An individual integrated circuit (IC) begins life as a silicon wafer, and goes through a sequence of lithography patterning, etching, deposition and ion implantation steps to transfer the chip design onto the substrate. A typical chip will require the lithography steps to be done 20, 30 and even 50 times before process completion.
Making higher density, faster, and more powerful chips depends upon innovations in light source technology. The drive for smaller feature sizes in chip design is enabled by the reduction of the light source wavelength used to lithographically pattern the chips.
Wafer Preparation and Cleaning
To prepare the wafer substrate for lithography exposure the surface of the wafer must be polished, cleaned and treated to ensure compatibility with the photoresist. Once the photoresist material is coated and soft-baked, the wafer is ready for the exposure step.
Exposure and Light Source
In providing the illumination for the exposure step, the light source is a critical element of the photolithography process. Leading edge manufacturing currently uses Deep Ultraviolet (DUV) excimer sources; Krypton Fluoride (KrF) at a wavelength of 248nm; and Argon Fluoride (ArF) at 193 nm. The recently introduced Extreme Ultraviolet (EUV) source delivers a wavelength of 13.5 nm.
In the lithographic exposure, the photo-resist coated wafer is exposed to a pattern of intense light projected from the light source. The light source illuminates the photomask, allowing the lithography pattern transfer to occur. In optical projection lithography, the minimum size of the features in the design is determined by the wavelength of the light source – the shorter the wavelength, the smaller the defined feature size can be.
During etching, the lithography mask pattern, which was transferred into the photoresist by the laser light, is further transferred into the substrate by a chemical process. The etch process selectively removes material from areas that are exposed by the photoresist. The final etched patterned fidelity reflects the quality of the lithography as defined by the light source; the more precise the lithography exposure, the more functional and capable the finished microchip will be.
Process Completion and Packaging
Following the final process steps, the wafer is electrically tested prior to being mounted and cut into die or chips. The die are prepared for packaging, final testing and shipment to customers. These microchips will then be used to power technology all around the world.