Microlithography and Nanolithography

Summary

Microlithography and nanolithography are two closely related fields in the area of electronics manufacturing. Microlithography is a process used in making integrated circuits in semiconductors. The process relies on the projection of an image onto a light-sensitive plate. Circuits made by microlithographic techniques can carry features as small as 100 nanometers in width. Nanolithography refers to the making of circuits and other features even smaller and finer than those possible through traditional microlithography. New developments in nanolithography are making it possible to apply the technology in areas beyond semiconductors, such as nanoelectromechanical systems (NEMS) in highly sensitive measuring devices.

Definition and Basic Principles

Some sources say that the difference between microlithography and nanolithography has to do with the size and precision of the electronic components being made. Microlithography is associated with a level of precision in manufacturing that can be measured in microns, while nanolithography is associated with nanometers.

In reality, the differences are more complicated than feature size. Microlithography is a term associated with the making of integrated circuits and semiconductors. It refers to a process of projecting images from a master pattern onto a light-sensitive surface. The process shares some basic traits with traditional lithography, a form of ink-based printing in which an image is transferred from a flat plate to a surface, such as paper or cardboard.

Nanolithography refers not just to microlithography on a smaller scale. It involves a wide range of technologies, including the use of electron beams and scanning probes, to make components in products ranging from video screens to cultures for the growing of organic tissue used in transplant surgery. While the terms are not used interchangeably, the fields are often grouped together, such as in the Journal of Micro/Nanolithography, MEMS, and MOEMS. (MEMS stands for microelectromechanical systems and MOEMS for micro-opto-electromechanical systems.)

Background and History

The roots of microlithography can be found in a program funded from 1957 to 1963 by the U.S. Army Signal Corps. The Micro-Module Program was launched shortly after the development of the transistor by scientists at Bell Telephone Laboratories in 1947. Before transistors, electronic components relied on vacuum tubes, which were large, broke easily, and generated heat. Transistors offered the possibility of making much smaller and more durable devices that needed less energy to run. However, transistors presented a number of manufacturing problems. They were made from numerous parts consisting of different materials that had to be assembled and soldered by hand. The Micro-Module Program encouraged the development of newer technologies in which components could be made with built-in wiring and joined together to form circuits. Texas Instruments engineer Jack Kilby discovered a way to make integrated circuits in a more cost-effective way in 1958. At nearly the same time, Robert Noyce at Fairchild Semiconductor made a related breakthrough using silicon transistors. Microlithography grew rapidly with the demand for microchips in electronics.

One of the earliest advances in the field of nanolithography was a 1983 NATO Advanced Research Workshop in Rome. Leading experts from a range of fields, including X-ray and electron-beam technology, met for a cross-disciplinary discussion of the ways in which nanolithography could be applied to technological problems. The field has since evolved beyond semiconductors to areas, such as chemistry and medicine.

How It Works

Microlithography is a process used specifically for the manufacturing of microchips. It allows a series of circuits to be placed on a hard surface with a tremendous amount of precision. Improvements in technology have made it possible for microchips to carry increasing numbers of circuits in new combinations and patterns.

Microlithography begins with the creation of a template, or pattern, known as a photomask. The photomask plays a role similar to that of a plate on a printing press. It carries an image of the circuits to be built on the surface of a chip. The image on the photomask is projected onto the chip's surface with the help of a lithographic lens. This lens is one of the most accurate and advanced pieces of equipment needed to make a microchip. An individual lens can be made out of more than thirty components and weigh almost a half-ton. A source of intense light, such as a high-pressure mercury arc lamp or a laser, is shone through a series of refraction elements and eventually through the photomask itself. The light projects the photomask's image onto the surface of a silicon wafer by shining through the lithographic lens, which condenses the image down to a suitably small size for placement on the surface of the wafer. The wafer's surface has been coated with a thin layer of a light-sensitive substance. When the image is shone onto this layer, known as a photoresist or simply a resist, the intensity of the light burns the image into the resist's surface within a fraction of a second. This procedure is known as an exposure.

A single microchip goes through many exposures in the manufacturing process. At each step, the accuracy of the procedure must be absolute, or the microchip will be unusable. The tolerance for errors in focusing and placing the image from the photomask is below 1 percent or a few nanometers. Wafers are held in place by a vacuum system, and their positions are verified by lasers before each exposure occurs. For a manufacturing process to be profitable, errors must be as low as possible while wafers pass through each stage quickly. A standard system might process fifty to one hundred or more wafers in an hour.

Unlike microlithography, there are a large number of manufacturing methods used in nanolithography, each with its own steps and specialized equipment. Some of these methods rely on radiation, such as X-rays or ultraviolet light rather than lasers. Electron projection lithography falls into a similar category. It relies on an electronic lens and electric and magnetic fields to guide charged particles into specific shapes and patterns with the help of a mask. Many of these particles move on wavelengths short enough to pass directly through a mask. In these cases, the masks deflect the particles away from the resist rather than stopping the energy entirely. Other methods do not use photomasks, which are expensive and difficult to produce for small manufacturing batches, but focus a beam of light directly onto the resist surface to make the needed patterns.

These methods belong to a category of nanolithography known as top-down. In contrast, bottom-up manufacturing methods use combinations of chemical solutions to assemble images directly on a surface. These images are so precise that the assembly process takes place on the scale of individual molecules.

Applications and Products

Semiconductors. Microlithography is heavily used in the semiconductor industry. Like printing with lithographic plates, microlithography allows a highly complex pattern of electronic circuits to be transferred to many individual microchips at the same time. This process makes it possible to invest a great deal of time and financial resources into the creation of a sophisticated technological design and then duplicate the design on a broad scale precisely and quickly.

The process of microlithography is only one set of steps in a chain that leads to a finished microchip. Before the surface of a wafer is ready to receive a microlithographic image, the wafer itself must be formed from highly refined silicon. After refining, silicon is formed into single-crystal rods known as ingots and sliced into wafers. The refining and wafer-slicing process requires such specialized technology that this stage is often handled by companies devoted exclusively to it.

The silicon wafers are then purchased by semiconductor manufacturers, which prepare the wafers for microlithography by adding silicon dioxide (a form of sand) and elements with specific electrochemical properties, such as boron, phosphorus, or arsenic. Silicon dioxide serves as an insulating base for the other additives, which are formed on the surface of the chip into transistors and circuits through repeated exposures from a photomask in the process of microlithography. The transistors are connected to each other through a metallization process that uses aluminum, tungsten, or a number of other metals and their compounds. The same metallization process is also used to make bonding pads on the surface of the chip. These pads are the points through which the chip communicates electronically with other chips and with the machine in which it will be installed.

Once the image on the chip's surface is complete, the surface itself is smoothed chemically and covered by a protective layer. The chip's transistors and circuits are tested before the chip is sold by the manufacturer. At this stage, the chip may be altered to a larger product's specifications, a process known as die cutting, before being installed inside another device.

Carbon Nanotubes. In contrast to microlithography, nanolithography as a manufacturing process has applications in a wide range of fields. Nanolithography has been found to be useful in the making of nanoelectromechanical systems (NEMS). One type of structure that shows the most promise is the carbon nanotube. Through nanolithography, carbon atoms are bonded together into molecules shaped like long, hollow tubes with closed ends. The result is no wider than a few nanometers but can be up to several inches long. Carbon nanotubes belong to a family of carbon molecules known as fullerenes, a group that also includes structures such as buckyballs. Because carbon molecules are strong conductors of electricity, nanotubes have many potential uses as components in transistors and circuits. They can also be used in electrical sensors that need a high level of precision and sensitivity, such as those used to track changes in gases.

The potential uses of carbon nanotubes are still being discovered. The presence of carbon atoms in compounds such as steel makes the resulting product lighter and stronger. For example, a company in Finland developed a way to add carbon nanotubes to the blades of windmills. The nanotubes make the windmill blades twice as light as those made from glass fibers and several times stronger, which allows for larger blades that move more efficiently. Other applications are still in the research stage. Scientists have noted, for example, that carbon nanotubes resemble tiny needles and may be used to carry antibodies and pharmaceuticals to highly targeted areas within the body, such as cancerous tumors.

Biosensors and Cell Biology. Dip-pen nanolithography (DPN) has shown great promise in the field of cell biology. DPN allows a chemical pattern of molecules, such as those found in the DNA of a cell, to be copied onto the surface of a microchip. DPN makes it possible to copy this pattern not once but thousands of times within a single manufacturing process. Unlike microlithography, which depends on light being shone through a photomask, DPN deposits a chemical agent directly onto the surface of a chip with the help of a "dip pen"—the highly precise tip of an atomic-force microscope. Direct contact between the pen and the surface means a higher possibility of problems, such as contaminating agents.

However, this challenge is outweighed by the possibilities presented by the technology in developing new kinds of cell-based therapies and other medical applications. Scientists at Northwestern University in Chicago found that DPN may be used to replicate electrodes in DNA patterns. With the information from these patterns, customized biosensors could be developed that could, in theory, be reintroduced into living cells. These sensors could then transmit data that would be used to monitor a body's vital functions. The sensors could also track the progress of a disease or drug therapy.

Advancements beyond the technology of DPN are already being pursued. Polymer-pen lithography (PPL) uses many of the same processes as DPN, but it involves larger arrays of pens—up to 11 million, according to an estimate from Northwestern—as well as the ability to push the pens with varying amounts of force against the writing surface. These features allow many details to be transferred to the writing surface during a single procedure, which makes the manufacturing process faster and more efficient. Beam-pen lithography (BPL) blends lithographic techniques with the technology behind scanning electron microscopes. These approaches are still in the early research stages, but they present possibilities for developing new treatments in fields ranging from genetics to heart disease.

Careers and Course Work

Microlithography and nanolithography are highly specialized career areas that involve the intersection of many academic disciplines, such as engineering, physics, and chemistry. Students interested in working in microlithography and nanolithography should pursue bachelor's degrees in electronics engineering, electrical engineering, or materials science. A student with a background in a related field such as mechanical engineering, physics, or computer science would also be well-positioned for a job in microlithography or nanolithography. Relevant coursework starts with a foundation in the physical sciences and mathematics. Depending on the institution, advanced coursework can be highly specialized. Several schools in the United States alone offer bachelor's degrees with majors in nanotechnology.

Due to the complex and specialized knowledge required to work in microlithography or nanolithography, many job candidates complete master's degrees or doctorates. Advanced degrees improve the earning potential of graduates in the field and prepare them for higher-level positions, such as research team leader. Some large research universities, such as the Georgia Institute of Technology, host interdisciplinary centers that allow students and faculty members to gain experience working with microlithography and nanolithography applications.

The job market for semiconductor manufacturing was minimal in the early twenty-first century. The US Bureau of Labor Statistics found that the number of jobs in the field fell sharply in the first decade of the twenty-first century due to increased efficiencies in manufacturing processes and offshoring manufacturing. Career opportunities increased in the 2020s as the US government invested in domestic semiconductor fabrication production.

Social Context and Future Prospects

The demand for new applications of microlithography and nanolithography is expected to continue for the foreseeable future. Microlithography remains one of the most precise and cost-effective ways to manufacture integrated circuits. Photolithography is the most commonly used microlithography method, though others, like electron-beam lithography, remain relevant. An ever-increasing number of consumer devices rely on microchips to function, ensuring the need for inexpensive components will remain for many years.

In 2021, a global shortage of semiconductor chips created a surge of work in 2022. The shortage was mainly caused by the COVID-19 pandemic when many people were working remotely and needed additional and faster technology. Chipmakers were also confined to their homes, causing production to cease temporarily. Additionally, the supply and demand for semiconductor fabrications by region have historically been imbalanced. In the early 2020s, the US consumed 34 percent and China 29 percent of the world's total supply. However, only 14 percent of the world's semiconductor supply was produced in the US and 21 percent in China.

Beginning in late 2021, semiconductors ramped up production to end the shortage. To capture more of the global market, US semiconductor firms invested about UDS$50 billion in research and development—the highest in history. This led to a hiring surge that continued the following year.

As production and demand increased, new technologies and manufacturing methods emerged, such as extreme ultraviolet lithography (EUVL). EUVL is a photolithography method that emerged in the late 2010s and early 2020s. It uses extreme ultraviolet light to manufacture integrated circuits.

The outlook for nanolithography is also optimistic. As new applications for the technology are discovered, specialists are increasingly needed. Some of the most promising areas of opportunity are in biotechnology, chemistry, and electronics outside of traditional integrated circuits. There is debate within the field about the level of precision that can be achieved in manufacturing through nanolithography. If the process is no longer cost-effective below a certain point, a next-generation technology must be developed.

Bibliography

Awan, Tahir Iqbal, et al. Chemistry of Nanomaterials Fundamentals and Applications. Elsevier, 2020.

Benisty, Henri, et al. Introduction to Nanophotonics. Oxford University Press, 2022.

Cao, Guozhong, and Ying Wang. Nanostructures and Nanomaterials: Synthesis, Properties, and Applications. 2nd ed., World Scientific Publishing, 2011.

Geng, Hwaiyu. Semiconductor Manufacturing. 2nd ed., McGraw-Hill Education, 2018.

Kumar, Ashok, et al. Nanotechnology for Advanced Biofuels: Fundamentals and Applications. Elsevier, 2023.

Levinson, Harry J. Principles of Lithography. 4th ed., SPIE Press, 2019.

Su, Zhi. "The 2022 SIA Factbook: Your Source for Semiconductor Industry Data." Semiconductor Industry Association (SIA), 5 May 2022, www.semiconductors.org/the-2022-sia-factbook-your-source-for-semiconductor-industry-data. Accessed 9 June 2022.

Suzuki, Kazuaki, and Bruce W. Smith, eds. Microlithography: Science and Technology. 3rd ed., CRC Press, 2020.

Teresa, José M. Nanofabrication: Nanolithography Techniques and Their Applications. IOP Publishing, 2020.

"U.S. Semiconductor Jobs Are Making a Comeback." The White House, 20 Mar. 2024, www.whitehouse.gov/cea/written-materials/2024/03/20/u-s-semiconductor-jobs-are-making-a-comeback. 10 June 2024.