Create Nano-Sized Features on Computer Chips Using Nanolithography
The semiconductor industry has been in the nanotechnology business for years. They use tools and processes to etch nano-sized patterns on silicon wafers coated with a material called photoresist. Those patterns make up the circuits on the chip that allow your computer to process data. The process used to make these patterns is called nanolithography.
The integrated circuits that are the brains of your computer include nano-sized structures. To create nano-sized features for integrated circuits on silicon wafers requires a machine called a stepper, which uses a technique called lithography to print a pattern on the chip. Microprocessors with a 32-nanometer feature size made with a nanolithography process have as many as 995 million transistors packed on one computer chip.
In a stepper, light shines through a reticle, or photomask, which contains the pattern to be printed, and a lens focuses the pattern on photoresist coating the surface of a semiconductor wafer. The wafer is then shifted, or stepped, so that an unexposed region of photoresist moves under the optical system, exposing that region using UV light. This stepping continues until the pattern is repeated across the entire wafer.
Lithography is similar to film photography, in which a pattern is exposed on photoresist and the photoresist is developed using photographic chemicals. The developing process in both cases washes away the unexposed photoresist, leaving the resist in the desired pattern on the wafer’s surface. An etching system removes the silicon and other layers that are not covered by the pattern of the photoresist.
Manufacturers keep coming up with techniques to reduce the minimum feature size they can print. The method currently used by most high volume integrated circuit manufacturers is called 193 nm immersion lithography. The 193 nm relates to the wavelength of ultraviolet light generated by a laser used to expose the resist, and immersion refers to the fact that you are immersing the lens in a puddle of ultrapure water.
Air between the lens and photoresist causes light to bend slightly, due to differences in the index of refraction between air and the lens. However, the index of refraction for water is closer to that of the lens, so the light bends less and the stepper can print a finer pattern.
When manufacturing integrated circuits, you can expose several different patterns on a wafer and each of these patterns defines a particular layer or type of material.
For example, one layer might define the metal lines that connect various components of the circuit, while another layer might define the gate of transistors in the circuit. (The gate of a transistor is the region that allows an applied voltage to turn the transistor on or off and is the smallest region to be patterned in the integrated circuit.)
Currently, manufacturers are working with steppers that use 193 nm immersion lithography to produce integrated circuits with a 32 nm minimum feature size.
Although the 193 nm immersion system becomes less inefficient as the feature size is reduced, manufacturers will have to use this system until the next-generation system is available. That next improvement in steppers and lithography will be a system that uses ultraviolet light with a 13.5 nm wavelength. This system is called extreme ultraviolet, or EUV, because it uses ultraviolet light with such an extremely short wavelength.
Extreme ultraviolet nanolithography systems don’t use immersion techniques. Instead, the light path and the wafers that are processed are in a vacuum because air or water would block the EUV beam.