Lithography — Painting Circuits with a Brush of Light

The Term "Lithography Equipment"

"Latest lithography equipment brought in for the first time," "Semiconductor industry in a race to secure lithography equipment." You may have seen headlines like these in semiconductor articles. The articles treat the arrival of a single machine as major news.

What lithography equipment does can be stated in a single sentence. Lithography is the process of shining light to transfer a circuit drawing onto silicon. The small switches inside a smartphone chip and the wires connecting those switches are all drawn this way.

This article carries a single brush. Light is the brush, and silicon is the drawing paper. Anyone who has ever drawn a picture with a brush knows a rule. The thicker the brush, the harder it is to draw a thin line. With a 1-centimeter-thick brush, it is hard to draw a line thinner than 1 centimeter.

What the semiconductor industry has done for decades is wrestle with this rule. It gathered the brush tip to be sharper, drew pictures that could not be drawn all at once by dividing them into multiple steps, and finally made a much thinner brush. The 1-centimeter brush is an example created by this article to aid understanding, and the numbers attached to the brush are also set by this article. We will carry this brush all the way to the end of the article.

Circuits Are Not Drawn Line by Line

Though we called it a brush, circuits are actually not drawn line by line. The entire design drawing is transferred and printed with light. The principle is similar to printing a roll of film onto photographic paper.

There are three materials. First is the drawing paper. A wafer is a thin, round silicon plate used to make chips. Multiple chips are made together on a single wafer.

Second is the original drawing. A mask is a plate containing the circuit's design drawing. Optical lithography works by first making this mask, and then projecting the mask's image onto a wafer coated with a film that reacts to light (the photoresist we will see right next) (DOI: 10.1109/jproc.2007.911853). The image is reduced in size as it passes through a lens. That is why the lines on the mask are drawn thicker than the lines on the wafer.

Third is the film that reacts to light. A photoresist is a thin film whose properties change when it receives light. It is commonly made of polymers, and metal oxide photoresists have also been developed for exposures using the shortest light we will look at later (DOI: 10.1039/c9ra08977b). The property that changes is the degree to which it dissolves in chemicals (DOI: 10.2533/chimia.1993.387). Therefore, when washed with chemicals, either only the areas that received light are washed away, or conversely, only the areas that received light remain.

Exposure takes place where these three materials meet. When light transfers the mask's image onto the photoresist, an image made of photoresist remains on the wafer. This photoresist acts as a shield. When the exposed areas not covered by the film are etched away, the image is transferred to the actual material underneath. Carving patterns into a chip is mostly done in these two steps of exposure and etching (DOI: 10.1109/jproc.2007.911853).

Let us follow a single line. A single circuit line is drawn on the mask. Light carries the image of that line and passes through a lens, shrinking the image. When the shrunk image hits the photoresist, the degree of solubility differs between the areas that received light and the areas that did not. After washing with chemicals, a single line made of photoresist remains on the wafer, and etching around it imprints the line onto the actual material. All the other lines contained in the mask follow the exact same path.

Chip circuits are not drawn by hand. They are made by transferring and printing a mask's image onto a photoresist using light.

The Brush Thickness Is the Wavelength of Light

Even pictures taken with light have a limit like a brush. No matter how sharp the lines of the mask are, the light touching the wafer blurs a little at the edges. This is because light travels while spreading like a wave.

What governs the size of that blur is the brush thickness of the light. The wavelength is the distance between one crest and the next crest in a wave of light. The wavelengths in this article are measured in nanometers. One nanometer is 1 over 1,000,000,000 of a meter. The shorter the wavelength of the light, the smaller the blur, allowing smaller pictures to be printed. Lithography equipment used for semiconductor mass production has continuously transformed into equipment using shorter-wavelength light to meet the requirement for resolution—that is, how sharply things as small as possible can be printed (DOI: 10.1143/jjap.49.030001).

Besides the brush thickness, there is one more thing that determines the thickness of a line. Even with the same brush, if you gather the tip of the brush to a sharp point, a thinner line comes out. In light, the lens does the job of gathering the brush tip. The numerical aperture is a value representing how wide an angle of light the lens gathers. It is written as NA for short in English. If it is equipment that gathers light with a mirror, the mirror does this job. Increasing the numerical aperture of the projection lens raises the resolution capability (DOI: 10.1116/1.1520577). The point that utilizing the full power of a short wavelength requires an optical system design with a larger numerical aperture comes from the same principle (DOI: 10.1364/iodc.1998.lwa.3).

Combining the two becomes the rule of this article. The thinnest line that can be drawn with light gets thinner as the wavelength is shorter and the numerical aperture is larger. Physics writes this relationship in a single equation, but in words, this is everything. Even with the same wavelength, lines smaller than the wavelength can be drawn depending on the lens and technique.

Gathering the brush tip comes with a price. The sharper the brush tip, the more accurately the distance to the drawing paper must be matched. In lithography, increasing the numerical aperture makes the depth where the focus matches shallow. Resolution and depth of focus are a trade-off relationship, which has also been confirmed by experiments measuring while changing the numerical aperture in lithography equipment where the space between the lens and the wafer is filled with liquid (DOI: 10.3390/mi13111971).

With Shorter Light and a Wet Brush

The industry first changed its brush. The wavelength of the light used for exposure shrank from 436 nanometers to 365, 248, and finally 193 nanometers. This sequence was taken because higher resolution was needed (DOI: 10.1364/eul.1994.elpm.10). The brush thickness became less than half.

The brush stopped at 193 nanometers for a while. After that, tricks to draw thinner lines with the same brush followed. The first trick is wetting the brush. A brush soaked in water gathers its tip more sharply than a dry brush.

Exposure has the same trick. Immersion lithography is a method that increases the numerical aperture by filling the space between the lens and the wafer with liquid instead of air. The purpose of filling the liquid is to increase the numerical aperture of the lens and raise the resolution capability (DOI: 10.1116/1.1520577). Immersion lithography greatly increases the numerical aperture to create smaller patterns (DOI: 10.1143/jjap.45.9280). The brush thickness was left at 193 nanometers, and only the brush tip was gathered more.

This trick also comes with a cost. As the numerical aperture grows, light enters the wafer more obliquely, and that much more light bounces back up after hitting the floor beneath the photoresist. The bounced light distorts the thickness of the printed lines. Therefore, a film is laid under the photoresist to reduce the bounced light, but light travels as waves vibrating in one direction, and the degree of bounce differs for each vibration direction. The thickness of this film must also be re-determined to match the direction in which the light vibrates (DOI: 10.1143/jjap.45.9280).

As a result of piling tricks layer upon layer, 193-nanometer light printed sizes much smaller than its wavelength (DOI: 10.1145/1862879.1862880, DOI: 10.1109/jproc.2007.911853).

Even with a brush of the same wavelength, gathering the brush tip produces a thinner line, and immersion is a trick that wets the brush to gather the brush tip.

If You Cannot Draw It All at Once, You Draw It in Parts

Let us say you are painting stripes with a 1-centimeter brush. Because the paint bleeds at the edges of the brushstrokes, suppose you must leave 4 centimeters from one line to the next when painting them all at once so the lines do not bleed together. If you want to draw the lines closer together than this, there is a way. First, you paint one line every 4 centimeters and let them dry. Then, you prepare to paint anew and paint one line right in the middle between those lines. Every time you paint, you keep the 4-centimeter spacing between lines, but when you are finished, the lines are placed one every 2 centimeters. With the same brush, you have cut the line spacing in half.

Lithography actually walked this path. Multiple patterning is a method where a single layer's pattern is printed in two or more separate steps. A 2009 commentary noted that even when adding all sorts of computational techniques to immersion, the method of printing all at once with 193-nanometer light was reaching its limit near the generation named "22 nanometers" (DOI: 10.1145/1862879.1862880). For the next generation, the "20-nanometer" generation, the solution used was printing in two separate steps using 193-nanometer immersion lithography (DOI: 10.1149/06001.0173ecst). What this "nanometer" name is actually the length of is covered separately in What Is "3 Nanometers" the Length Of? — Measuring a Chip's Name with a Tape Measure.

Drawing in parts did not stop at two times. Double patterning became a technology that extended the lifespan of 193-nanometer immersion lithography. Instead of printing separately, there is also a method where you coat a film next to a once-printed line and etch it away to increase the number of lines, and methods even emerged that repeat the same principle to increase the number of lines threefold or fourfold (DOI: 10.1149/06001.0273ecst).

The price is paid in two places. The first is alignment. If the second line does not fall precisely in the exact middle, the line spacing becomes uneven. In the method of printing in two separate steps, how accurately the two patterns are overlapped is one of the core metrics that decides success (DOI: 10.1149/06001.0173ecst). The second is the labor. The method of printing in two separate steps involves printing and etching, then printing and etching again (DOI: 10.1149/06001.0173ecst). The method of printing once and multiplying lines with films is also cited as having the drawback of requiring the processes of coating and etching films to be chained together multiple times (DOI: 10.1116/1.4963194).

A pattern that cannot be drawn at once can be drawn in parts, but the more you divide it, the more the process grows and the more accurately the patterns must overlap.

A Thinner Brush — Extreme Ultraviolet

No matter how much know-how accumulated, the brush thickness stayed the same. The industry eventually changed the brush. Extreme ultraviolet is light with a very short wavelength, and the extreme ultraviolet used for lithography has a wavelength of 13.5 nanometers. Its English abbreviation is EUV. 13.5 nanometers is roughly one-fourteenth of 193 nanometers (DOI: 10.1039/c9ra08977b). It is like a 1-centimeter brush becoming a 0.7-millimeter brush.

However, this only compares the brush thickness, that is, the wavelength. The line thickness is also determined by how tightly the brush tip is gathered, that is, the numerical aperture. Lithography equipment filled with liquid uses an optical system with a numerical aperture greater than 1 (DOI: 10.1364/ao.457833), whereas the numerical aperture of extreme ultraviolet equipment is smaller than that (DOI: 10.1364/iodc.1998.lwa.3). Therefore, the lines that can be drawn do not become one-fourteenth as thin, and the difference in line thickness is much smaller than the difference in brush thickness.

Extreme ultraviolet lithography was anticipated as a technology to succeed 193-nanometer immersion and multiple patterning at the finest layers (DOI: 10.1515/aot-2017-0029). One review article noted that extreme ultraviolet can replace that combination in some cases (DOI: 10.1039/c7cs00080d).

Using a thinner brush meant changing how the brush is held in the first place. Once the wavelength becomes this short, the methods used until then must be fundamentally changed (DOI: 10.1002/vipr.200390020). Unlike conventional lithography, extreme ultraviolet lithography is a reflective device that gathers light using mirrors coated with multiple layers of films instead of lenses (DOI: 10.1142/s0219607707000219). These multi-layered reflective films are cited as the core technology that made extreme ultraviolet lithography possible (DOI: 10.1364/eul.1996.rmc162). An extreme ultraviolet equipment design introduced in 1999 had the mask also reflect light, and reduced and focused the image to a quarter using mirrors (DOI: 10.1109/3.760315).

Creating the light was also different. Equipment for mass production uses this method instead of a bulb. When tin is struck with a laser to make it extremely hot, that tin emits this light (DOI: 10.1063/1.2906901). Such a light source that creates light with a laser was installed and operated in chip company factories (DOI: 10.1515/aot-2017-0029). One non-peer-reviewed preprint manuscript notes that the body of the latest equipment is placed inside a high vacuum, and when 50,000 tiny tin droplets fall per second, a laser strikes each droplet twice (DOI: 10.31224/7350). It is the task of accurately hitting a droplet in every gap obtained by dividing 1 second into 50,000 pieces.

This light was too dim for a long time. Even in 2014, the target output was 200 watts, but the light produced by the equipment at the time was 40 to 55 watts (DOI: 10.1063/1.4863412). This is roughly one-fifth to one-fourth of the target. When the light is dim, it takes a long time to print a single wafer, and the number of wafers printed per hour is a decisive factor determining cost (DOI: 10.1016/j.crhy.2006.10.002).

Extreme ultraviolet lithography was first proposed in 1988 (DOI: 10.1063/1.4863412), and thereafter walked the path of commercialization over decades (DOI: 10.51593/20240003). It began to be used for mass production in 2019, starting with logic chips that handle calculations, and the following year expanded to logic chips of a yet finer generation process (DOI: 10.1364/euvxray.2022.ew6a.6).

Extreme ultraviolet is light with a brush thickness one-fourteenth the size, but to use that brush, lenses had to be replaced with mirrors, and lightbulbs with tin struck by lasers.

The Next Brush Tip and the Questions That Remain

The step after extreme ultraviolet is gathering the tip of the brush. The wavelength is kept at 13.5 nanometers while increasing the numerical aperture. The point that using the full power of short wavelengths requires a design with a larger numerical aperture was already made back in 1998 (DOI: 10.1364/iodc.1998.lwa.3). The industry calls extreme ultraviolet equipment with an increased numerical aperture "High-NA" (DOI: 10.1364/ao.555171). It means the numerical aperture, or NA, is high. Just as we wetted a 1-centimeter brush and gathered its tip earlier, this time we gather the tip of a 0.7-millimeter brush even further.

As the brush gets thinner, new problems are revealed. A thin brush tip holds only a little paint, and when there is very little paint, the lines become blotchy. Light has been compared to a brush so far, but in this problem, the amount of light hitting a spot on the photoresist corresponds to the paint. Light arrives in units of very small grains. When the spot is very small, the number of grains hitting that spot is small, so some spots randomly get many, and other spots randomly get few. The chemical reactions inside the photoresist also vary slightly from spot to spot. Simulations have tested printing very small holes on the photoresist—namely, spots connecting upper and lower layers—and changed the size of the holes where both were successfully printed, leading the authors to write that the chemistry and processing of the photoresist must be controlled very precisely (DOI: 10.1116/1.1629294). When the size is small, this fluctuation leads to printing failures where lines break or stick together, lowering the proportion of usable chips (DOI: 10.1117/1.jmm.17.4.041011).

Even in research printed with experimental high-numerical-aperture extreme ultraviolet equipment, suppressing the effects of randomness that occur during the process of washing away the photoresist was cited as important to print smaller lines (DOI: 10.7567/jjap.55.096501). One simulation study concludes that the main cause of defects was the jaggedness of light, and that such failures cannot be reduced simply by improving the photoresist material (DOI: 10.35848/1347-4065/ac5b22). How to reduce the blotches caused by a shortage of paint as the brush gets thinner remains an open question.

Reading Article Headlines with a Brush

Let us return to the article headlines from the beginning. The two headlines below are hypothetical examples created for this article.

The first headline is "First Introduction of High-NA Extreme Ultraviolet Lithography Equipment." Did the brush become thin? The wavelength remains 13.5 nanometers as before. What changed is the tip of the brush, namely the numerical aperture. And even in experiments with equipment that increased the numerical aperture, the accidental jaggedness from washing away the photoresist was noted as a separate task to solve.

The second headline is "Fine Processing Through Multiple Patterning Without Extreme Ultraviolet." The brush remains 193 nanometers as before. The brush tips are wet and gathered, and a single layer is painted multiple times in portions. When reading this headline, one can recall the facts that the process increases as much as it is divided, and the divided pictures must overlap precisely.

If an article features lithography equipment, you can ask three things: Did the brush become thin? Did they gather the brush tips? Did they paint by dividing multiple times? The tricks looked at in detail in this article are all either one of these three or a combination of them.

Where to Look Further

This article traced only the main stream of photolithography with a single brush. The deliberate distortion of mask patterns for correction, the trade-off in depth of focus, the difficulties of extreme ultraviolet masks and light sources, line edge roughness, and random defects are covered next in "Printing with Light — How Lithography Draws Smaller Than Its Own Wavelength".

When the "nanometer" attached to generation names drifted apart from actual length is looked at by "After Moore's Law — What a "Nanometer" Came to Mean".

Other articles in "Chips from Scratch" are "A Transistor Is a Switch — A Tiny Faucet You Open and Close with Electricity", which covers what switches inside chips are; "What Is "3 Nanometers" the Length Of? — Measuring a Chip's Name with a Tape Measure", which unpacks the name "3 nanometers"; and "Fetching Is Slower Than Computing — The Memory Bottleneck and HBM", which covers why moving data is slower than computing.

Source Papers

• "Lithography and Other Patterning Techniques for Future Electronics" (2008) — DOI: 10.1109/jproc.2007.911853 — Optical lithography makes a mask, projects that image onto a photoresist-coated wafer, and patterning consists of two steps: exposure and etching.
• "Extreme ultraviolet lithography" (1999) — DOI: 10.1109/3.760315 — Extreme ultraviolet lithography uses a laser-produced plasma light source, a reflective mask, and a 4x reduction all-reflective imaging system.
• "Photolithography at the Limits of Optical Resolution" (1993) — DOI: 10.2533/chimia.1993.387 — Photolithography is the process of exposing a photoresist—whose developer solubility changes when it receives light—according to a pattern.
• "Radiation Chemistry in Chemically Amplified Resists" (2010) — DOI: 10.1143/jjap.49.030001 — High-volume exposure equipment has been repeatedly replaced with shorter-wavelength equipment to meet resolution requirements.
• "Effect of high numerical aperture lens on lithographic performance in 157 nm lithography" (2002) — DOI: 10.1116/1.1520577 — Immersion was studied to raise resolving power by increasing the projection lens numerical aperture, and high-numerical-aperture optical systems improve resolution and process latitude.
• "High numerical aperture designs for EUV projection lithography" (1998) — DOI: 10.1364/iodc.1998.lwa.3 — EUV optical systems at the time had a numerical aperture of 0.1, failing to fully utilize the resolution potential of the short wavelength, and higher numerical aperture designs are needed.
• "A Study on the Resolution and Depth of Focus of ArF Immersion Photolithography" (2022) — DOI: 10.3390/mi13111971 — Measured resolution and depth of focus according to numerical aperture in ArF immersion equipment to confirm the trade-off relationship between the two.
• "193 nm—Moderately EUV" (1994) — DOI: 10.1364/eul.1994.elpm.10 — 193 nm, following 436, 365, and 248 nm, is a natural extension driven by higher resolution demands.
• "Single Anti-Reflection Coating Optimization with Different Polarizations for Hyper Numerical Aperture Immersion Lithography" (2006) — DOI: 10.1143/jjap.45.9280 — Immersion creates smaller patterns with a hyper-numerical aperture, and as the numerical aperture grows, the angle of incidence increases, raising substrate reflectivity.
• "What is double patterning lithography and its impact on nanometer design?" (2009) — DOI: 10.1145/1862879.1862880 — Printing things much smaller than the wavelength with 193 nm is a fundamental challenge, and even with immersion and computational techniques added, single-exposure 193 nm reaches its ultimate limit near the 22 nm node.
• "Double Patterning Lithography Alignment and OVL-Control Strategy for 20nm Technology" (2014) — DOI: 10.1149/06001.0173ecst — 193 nm immersion double patterning was adopted as the solution for the 20 nm node, and overlay control is one of the key indicators of success.
• "(Invited) Optical Lithography Extension Technique with Multiple-Patterning Process" (2014) — DOI: 10.1149/06001.0273ecst — Double patterning became a technology that extends the lifespan of 193 nm immersion exposure, and there are self-aligned double, triple, and quadruple patterning.
• "Electron beam lithography patterned hydrogen silsesquioxane resist as a mandrel for self-aligned double patterning application" (2016) — DOI: 10.1116/1.4963194 — Self-aligned double patterning has only one exposure step, but requires multiple sequential deposition and etch steps.
• "Review of recent advances in inorganic photoresists" (2020) — DOI: 10.1039/c9ra08977b — A promising light source for next-generation lithography is 13.5 nm (92 eV) extreme ultraviolet, and metal oxide photoresists have been proposed and developed for EUV.
• "Light sources for high-volume manufacturing EUV lithography: technology, performance, and power scaling" (2017) — DOI: 10.1515/aot-2017-0029 — EUV lithography is expected to succeed 193 nm immersion multi-patterning at core layers below 10 nm, and second-generation laser-produced plasma light sources have been installed and operated at chip companies.
• "Extreme ultraviolet resist materials for sub-7 nm patterning" (2017) — DOI: 10.1039/c7cs00080d — EUV is regarded in some cases as a potential alternative replacing the combination of ArF immersion and multiple patterning.
• "EUV‐Lithographie für zukünftige IC‐Chips" (2003) — DOI: 10.1002/vipr.200390020 — Optical lithography achieved miniaturization mainly by moving to shorter wavelengths, and the large wavelength reduction to extreme ultraviolet requires fundamental changes to existing methodologies.
• "EUV lithography for semiconductor manufacturing and nanofabrication" (2007) — DOI: 10.1142/s0219607707000219 — Unlike conventional optical lithography, EUV lithography is a reflective optical system using multilayer mirrors.
• "High-NA EUV Lithography" (2026, preprint) — DOI: 10.31224/7350 — Structural explanation of 0.55 numerical aperture EUV equipment: a high-vacuum main body where a laser strikes 50,000 tin droplets twice per second to create light.
• "Reducing dose sensitivity in high-numerical-aperture extreme ultraviolet lithography using an innovative source optimization method" (2025) — DOI: 10.1364/ao.555171 — Proposed a source optimization method for high-numerical-aperture extreme ultraviolet lithography (high-NA EUVL) equipment and showed performance stability improvement through simulations.
• "Reflecting Multilayer Coatings for EUV Projection Lithography" (1996) — DOI: 10.1364/eul.1996.rmc162 — Multilayer reflective coatings are the core technology that enabled EUV projection lithography, with challenges in physics, deposition, and metrology.
• "Design of an objective for a hyper-numerical-aperture immersion lithography tool with a multi-step alternative grouping design method" (2022) — DOI: 10.1364/ao.457833 — Designed a catadioptric objective optical system with a numerical aperture of 1.35 for immersion lithography equipment.
• "EUV lithography" (2006) — DOI: 10.1016/j.crhy.2006.10.002 — Wafer throughput is a decisive cost factor, so light source power, photoresist sensitivity, and system design must be considered together, and reliable operation of light source power has not yet been proven.
• "Tin laser-produced plasma source modeling at 13.5nm for extreme ultraviolet lithography" (2008) — DOI: 10.1063/1.2906901 — Laser-produced tin plasma is a proposed solution for the 13.5 nm light source in EUV lithography.
• "Extreme ultraviolet lithography and three dimensional integrated circuit—A review" (2014) — DOI: 10.1063/1.4863412 — A review of EUV lithography proposed in 1988. Compared to the intermediate focus light source power target of 200 W, equipment at the time produced only about 40 to 55 W.
• "Tracing the Emergence of Extreme Ultraviolet Lithography" (2024) — DOI: 10.51593/20240003 — The decades-long commercialization path of EUV lithography from the 1980s onward.
• "R&D Activities of EUV Lithography at NewSUBARU, and Possibility of Beyond EUV" (2022) — DOI: 10.1364/euvxray.2022.ew6a.6 — EUV lithography began to be used for mass production of logic devices at the 7 nm node in 2019 and the 5 nm node in 2020.
• "Statistical limitations of printing 50 and 80 nm contact holes by EUV lithography" (2003) — DOI: 10.1116/1.1629294 — Both fluctuations in light intensity and spatial distribution and local chemical variations alter contact hole size, requiring extraordinary control over photoresist chemistry and processes.
• "Stochastic printing failures in extreme ultraviolet lithography" (2018) — DOI: 10.1117/1.jmm.17.4.041011 — Stochastic effects lead to printing failures when sizes are small, significantly impacting yield.
• "Analysis of line-and-space resist patterns with sub-20 nm half-pitch fabricated using high-numerical-aperture exposure tool of extreme ultraviolet lithography" (2016) — DOI: 10.7567/jjap.55.096501 — Printed sub-20 nm half-pitch using high-NA EUV equipment, and suppressing stochastic effects in the development process is important for 11 nm half-pitch.
• "Applying stochastic simulation to study defect formation in EUV photoresists" (2022) — DOI: 10.35848/1347-4065/ac5b22 — In simulations, photon shot noise dominated defect formation, and such failures are not mitigated by material improvements alone.