photolithography

photolithography
Name	Photolithography
Caption	A silicon wafer undergoing processing in a cleanroom.
Uses	Integrated circuit fabrication, printed circuit board manufacturing, MEMS device creation

photolithography is a fundamental process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It employs light to transfer a geometric pattern from a photomask to a light-sensitive chemical photoresist on the substrate. The technique is essential for manufacturing integrated circuits, microelectromechanical systems, and other nanotechnology devices, forming the backbone of modern electronics and semiconductor device fabrication.

Overview

The origins of the technique are deeply intertwined with developments in photography and the printing industry. Its adaptation for semiconductor manufacturing was pioneered by companies like Fairchild Semiconductor and Intel during the rise of the Silicon Valley technology sector. The process enabled the Moore's Law trajectory of scaling down transistor sizes, a progression tracked by organizations like the International Technology Roadmap for Semiconductors. Modern advanced systems are produced by corporations such as ASML, Nikon, and Canon.

Process steps

The sequence begins with surface preparation, where a substrate, typically a silicon wafer, is cleaned and coated with a layer of photoresist using a spin coater. The coated wafer is then soft-baked on a hotplate to drive off solvent. Exposure follows in a machine called a stepper or scanner, where light projected through a photomask chemically alters the resist. This exposure tool is a key component of the semiconductor fabrication plant. After exposure, the wafer undergoes post-exposure bake before development, where a chemical solution, such as tetramethylammonium hydroxide, removes either the exposed or unexposed resist regions. The resulting pattern is then hard-baked to increase durability for subsequent processes like etching or ion implantation.

Photomasks and resolution

The photomask, or reticle, is a critical element defining the pattern. Historically made from glass plates coated with chromium, modern masks are fabricated using electron-beam lithography systems from companies like JEOL. The resolution, or minimum feature size, is governed by the Rayleigh criterion, which depends on the wavelength of the light source and the numerical aperture of the projection lens. This drove the industry's shift from g-line to i-line mercury lamps, then to deep ultraviolet lasers from Cymer, and now to extreme ultraviolet lithography using plasma sources. Techniques like phase-shift masking and optical proximity correction, developed by firms such as Mentor Graphics, are employed to push resolution limits.

Materials and chemistry

Photoresists are complex chemical formulations. Positive resists, which become soluble upon exposure, often use a photoactive compound like a diazonaphthoquinone derivative. Negative resists, which become insoluble, may rely on cross-linking polymers. The chemistry evolved significantly for Krypton fluoride and Argon fluoride laser wavelengths, requiring new platforms like chemically amplified resists pioneered by IBM. Other essential materials include anti-reflective coatings to prevent interference, adhesion promoters like hexamethyldisilazane, and developers supplied by companies like Tokyo Ohka Kogyo. The handling of these materials occurs within the stringent environmental controls of a Class 1 cleanroom.

Applications

Beyond its primary role in creating microprocessors and memory chips for companies like Samsung and Taiwan Semiconductor Manufacturing Company, the process is vital for manufacturing microelectromechanical systems used in sensors and actuators. It is also used in the production of printed circuit boards, flat panel displays, and photovoltaic cells. In biotechnology, it enables the fabrication of lab-on-a-chip devices and DNA microarrays. The technique is also employed in creating diffractive optical elements and certain types of MEMS-based projection display systems.

Challenges and limitations

As feature sizes approach atomic scales, fundamental physical limits present major hurdles. These include the diffraction of light, addressed by the multi-billion-dollar development of extreme ultraviolet lithography by ASML in collaboration with TRINAMIC and the U.S. Department of Energy. Other challenges involve stochastic variation in photon and resist chemistry interactions, and the immense cost of next-generation tools, impacting the economics of fabs like those operated by GlobalFoundries. Materials science challenges include finding suitable resists and pellicles for new wavelengths, while yield management requires sophisticated inspection tools from KLA Corporation. Environmental and safety concerns also persist regarding the use of chemicals like ethylene carbonate and the generation of perfluorooctanesulfonic acid compounds.

Category:Semiconductor device fabrication Category:Lithography (microfabrication)