Photoresist first appeared in human history nearly 200 years ago.At the Harry Ransom Center at the University of Texas at Austin, a world-class humanities and arts archive, there is a metal plate quietly leaning in one of the display cases, measuring 20 centimeters long and 16.5 centimeters wide.Most of the plate’s surface is shadowed, with only a white area exposed in the upper middle, outlining some contours of the shadowed parts. Upon closer inspection, it resembles two tall houses flanking a shorter one.This is the first photograph in human history—”View from the Window at Le Gras.”In the summer of 1826, in a small town in the Burgundy region of central France, 61-year-old Joseph Nicéphore Niépce (hereafter referred to as Niépce) was carefully applying asphalt evenly to a tin plate.Niépce had served in the French army during the Napoleonic era and had shown a keen interest in invention from a young age. After retiring, he returned to his birthplace in Burgundy and focused his efforts on researching photographic techniques.The tin plate coated with asphalt was placed inside a wooden box, which had a hole the size of a cup opening on the front. The sunlight was abundant that day, and the light passed through the hole in the wooden box, directly illuminating the tin plate.Asphalt is a chemical mixture that contains a large amount of polycyclic aromatic hydrocarbons, which naturally absorb ultraviolet light well. When exposed to ultraviolet light, the polycyclic aromatic hydrocarbons undergo a cross-linking reaction, which can be understood as the originally loose small molecular chains connecting hand-in-hand under light, forming a network-like molecular structure.This cross-linked molecular structure makes the asphalt harder, less soluble, and more resistant to acid and alkali corrosion. The areas of asphalt that were not exposed to ultraviolet light do not undergo cross-linking and remain in their previous state, making them easy to dissolve.The wooden box faced the window and was continuously exposed for 8 hours until dusk. Niépce took the tin plate out of the box and immersed it entirely in lavender oil, causing part of the asphalt to dissolve and fall off the plate.In the end, the parts of the asphalt that were exposed to light and hardened remained on the tin plate, while the areas that did not receive light were washed away.When Niépce took the tin plate out of the lavender oil, he was delighted to find that the outline of the scenery outside the window had been “etched” onto the asphalt by light.Five years before Niépce captured the first photograph in human history, in 1821, a 21-year-old young man graduated from Trinity College, Cambridge, named William Henry Fox Talbot (hereafter referred to as Talbot).Talbot’s father had passed away before he was born, and lacking a father figure, he was shy from a young age. However, it was this introverted nature that led Talbot to spend his time delving into various scientific knowledge, especially laying a solid foundation in mathematics and optics.In 1833, Talbot traveled to Lake Como in northern Italy, a glacial lake at the foot of the Alps. The lake water was as clear as a mirror, reflecting the magnificent Alps on both sides.As he gazed at the beautiful scenery before him, Talbot had a thought: could he invent a method to perfectly record the scenery he saw and preserve it as an image?With this thought in mind, Talbot pondered repeatedly and suddenly realized the essence of an image: from a natural perspective, an image is the projection of light on a specific medium, or in other words—light from the three-dimensional world captured on a two-dimensional plane.This profound thought established Talbot’s status as the “father of modern photography.”Upon returning to England, Talbot began researching how to present images on photosensitive materials. After several improvements, he used silver iodide as the photosensitive material and paper as the medium.Next, he placed the paper containing silver iodide into a camera for exposure. The areas exposed to light would decompose the silver iodide, reducing it to tiny metallic silver particles, thus forming a latent image. These silver particles formed an image that was very faint and almost invisible to the naked eye, which Talbot referred to as a “latent image.”Then, Talbot immersed the exposed paper in a developing solution made of silver nitrate and gallic acid. The “latent image” in the exposed area began to appear, turning into a visible, dark image; this step is called “development.”To prevent the paper from continuing to darken, he needed to remove the unexposed silver iodide. Talbot used sodium thiosulfate as a solution to dissolve the unexposed silver iodide, thus forming a stable image; this step is called “fixing.”After completing the above steps, the final result was a “negative”—the bright parts of the image appeared dark (due to the aggregation of silver particles), while the dark parts appeared light (the original color of the paper). In other words, the brightness relationship of the negative is the opposite of the real scene.Now we come to the final step—producing a positive image, which presents an image with brightness consistent with the real scene.This step is simple: tightly adhere the produced negative to another piece of photosensitive paper and expose it again. The light passing through the light areas of the negative darkens the corresponding areas of the positive below; while the dark areas of the negative block the light, leaving the corresponding areas of the positive unchanged.Finally, repeating the same development and fixing steps as for the negative yields a “positive image,” where the brightness relationship of the image matches the real scene.In 1841, Talbot named the entire process from producing a negative to replicating a positive the “Calotype process.”Unlike Niépce’s method of image production, the Calotype process was the first reproducible photographic technique in human history.Talbot’s most revolutionary impact was achieving the reproducibility of images based on Niépce’s foundation. He opened the last node for the underlying framework of photolithography, establishing the foundational concept of photolithography as “light → chemical change → pattern transfer → replication.”Both Niépce and Talbot used photosensitive materials that shared a common feature: after exposure, the materials hardened, making them less soluble, which is precisely the core chemical mechanism of negative photoresist.You might be curious, how did the prototype of positive photoresist come about?In the second half of the 19th century, Europe was in the throes of the Second Industrial Revolution. Industries such as machinery, railways, and construction began to develop rapidly, creating a huge technical demand—how to quickly, cheaply, and accurately mass-produce engineering drawings?Although the process of transferring from negative to positive was already mature at that time, there were still many unsatisfactory aspects, such as:
- The entire process required two exposures, two developments, and two fixings, which not only took a long time but also had a high error rate and cost;
- Most negatives at that time used paper as the medium, and the light yellow base color of the paper led to poor brightness in the printed positives;
- Negatives would lose detail after multiple transfers, resulting in unstable image quality.
Therefore, the industrial sector urgently needed a technology that could directly generate positives, was easy to operate, fast, and cost-effective.In 1855, a new graduate student named Johann Peter Gries (hereafter referred to as Gries) arrived at the University of Marburg in central Germany. This 26-year-old young man was born into a farming family in northern Germany, where his father hoped he would study agriculture, but Gries had no interest in agriculture and was particularly fond of organic chemistry.Three years later, Gries left Marburg to work as a teaching assistant at the Royal College of Chemistry in England. The dean at that time was also a German and an outstanding chemist in the field of organic chemistry, named Hofmann.Hofmann often encouraged students to conduct free experiments, which provided fertile ground for Gries’s discoveries.In 1858, while studying the reaction of aromatic amines with nitrites, Gries aimed to obtain better nitration reactants to explore the transformation pathways of aromatic amines. At that time, this was a field that few had explored.In a low-temperature environment, Gries slowly added a nitrite solution to an acidic aromatic amine solution. After the reaction, he did not obtain the expected colored precipitate but instead produced a clear solution.When he added another aromatic amine to this clear solution, it would instantly burst into extremely bright and vivid colors, usually yellow, orange, or red.If Gries had merely recorded this color reaction, he might have just discovered an interesting phenomenon, but his greatness lay in the subsequent in-depth research.Gries realized that there must be a highly reactive and unstable intermediate present in the initially generated clear solution. Through multiple experiments, he successfully isolated this intermediate, which was a completely unknown substance at the time.This substance appeared salt-like, could crystallize, and was very soluble in water. At the same time, it was extremely sensitive and would decompose upon heating, releasing a large amount of nitrogen gas.Gries also discovered that this substance would accelerate decomposition when exposed to light. He keenly realized that these compounds might represent a brand new nitrogen functional group system.In the following experiments, he began systematically studying the reaction mechanisms of this compound and, in 1858, published a paper in which he first proposed the concept of “diazo compounds,” one of the most important papers in the history of organic chemistry.Based on Gries’s discovery, the German company Kalle utilized diazo compounds as photosensitive materials to develop diazo copying paper. The copies produced from diazo copying paper were identical to the original images and developed faster, with lower material costs.This solution perfectly addressed the core demand for the mass, rapid, and precise replication of engineering drawings, allowing multiple departments to work simultaneously based on the same version of the drawings, greatly enhancing collaboration efficiency and accuracy.As the story unfolds, we find that by the 19th century, the theoretical foundations and product prototypes of negative and positive photoresists had already emerged, but their true entry onto the historical stage of the semiconductor industry occurred in the second half of the 20th century.In 1947, Bell Labs invented the transistor, ushering in the era of solid-state electronics. Then, in 1958 and 1959, Jack Kilby of Texas Instruments and Robert Noyce of Fairchild Semiconductor independently invented the integrated circuit, triggering another revolution in the electronics industry.At that time, transistors were transitioning from manual manufacturing to integrated production, and the entire semiconductor industry faced a tricky problem: how to accurately define the position and size of transistors on very small silicon wafers.In the 1950s, Kodak was the world’s strongest photochemical company. They had decades of experience in silver salt photography, photosensitive resins, developing chemicals, and film coating technology.In Kodak’s headquarters laboratory in New York, an internal R&D project was underway, aimed at developing higher resolution industrial photosensitive coatings. The initial intention of developing this product was not for semiconductors but for microphotography and printing plate industries.Kodak engineers experimented with various formulations and accidentally discovered a clever combination: using cyclized rubber as the film-forming material and hydrazone as the photosensitizer. During exposure, hydrazone absorbs ultraviolet light, generating free radicals that trigger cross-linking reactions in the rubber.After cross-linking, solubility decreases, making the exposed area insoluble, while the unexposed area remains soluble. After development, only the exposed area remains.In 1958, Kodak officially named this product: Kodak Thin Film Resist (abbreviated as KTFR). Kodak originally intended to sell KTFR as a precision industrial coating, but in 1959, semiconductor engineers unexpectedly discovered KTFR’s exceptionally stable performance and used it in photolithography.KTFR enabled integrated circuits to be produced on a large scale for the first time, transitioning chip manufacturing from manual experimentation to industrial mass production, marking the world’s first commercial negative photoresist used in semiconductor manufacturing.In the early days of the semiconductor industry, negative photoresist was the absolute mainstream.Entering the 1970s, integrated circuits continued to develop towards large-scale integration following Moore’s Law. The number of transistors that needed to be integrated on chips increased dramatically, requiring the line width of circuit patterns to be continuously reduced.In the wave of industrial development, the defects of negative photoresist began to surface. During the development process, organic solvents would seep into the already cross-linked polymer network of negative photoresist, causing the film to swell like a sponge.This phenomenon, known as “swelling,” affects the accuracy of the developed image. In severe cases, the edges of the image become blurred, and dimensions are distorted. It is like using a brush that has absorbed too much ink to draw a fine line, which will inevitably smudge.The “swelling” characteristic fundamentally limits the resolution of negative photoresist, making it difficult to lithograph fine patterns smaller than 2 micrometers.As the line width of integrated circuits approached the sub-micron level, the contact lithography machines and negative photoresist faced severe challenges.In 1973, PerkinElmer introduced the industry’s first projection lithography machine. This new design used a complex lens system to reduce and accurately project the patterns from the mask onto the wafer.Projection lithography machines overcame diffraction effects, achieving higher resolution, and their non-contact design greatly extended the lifespan of masks. Once launched, projection lithography machines quickly became the mainstream equipment favored by wafer manufacturers.In the development of semiconductor technology, materials and equipment must evolve in sync; any lag in either side can become a bottleneck for industry progress.The emergence of projection lithography machines highlighted the resolution limits of negative photoresist, thus giving rise to the “high-precision lithography alliance” composed of positive photoresist, which has dominated the semiconductor industry since the 1970s.The aforementioned German company Kalle, known for inventing diazo copying paper, had long held a leading position in the copying materials field. After World War II, Kalle merged into the Hoechst AG group, which was once one of Germany’s largest chemical groups.As a subsidiary of Hoechst AG, Kalle continued to focus on imaging chemical products and began developing new types of photosensitive resins. In the 1950s, Kalle developed a special formulation using phenolic resin (Novolak) as the film-forming material and diazo naphthoquinone (DNQ) as the photosensitizer.Researchers found that after exposure, diazo naphthoquinone (DNQ) would undergo photodecomposition and generate carboxylic acid, which would disrupt the hydrogen bonds in phenolic resin (Novolak), dramatically increasing the alkaline solubility of the resin in the exposed area.Based on this discovery, after repeated experiments and adjustments, Hoechst AG launched the world’s first positive photoresist in 1962, named AZ-15 (AZ comes from the German word Azid, meaning diazo).Initially, AZ-15 was used in Germany for printing industry plate making, mask making, and printed circuit boards, and by the mid-1960s, American researchers discovered it could also be applied to semiconductor photolithography.To enter the then-largest semiconductor market in the world, Hoechst AG made a very shrewd business decision: they collaborated with the American company Shipley, which had a foundation in plating chemicals and was familiar with the semiconductor industry.Hoechst AG provided the core photoresist preparation technology, while Shipley was responsible for marketing and sales in the United States. In 1965, they jointly launched an improved version of AZ-15 in the U.S. market—AZ-1350.As projection lithography machines gradually became popular in the 1970s, positive photoresist began to replace negative photoresist due to its lack of “swelling” phenomenon, achieving higher resolution and more vertical and precise pattern edges, becoming the mainstream photoresist for G-line (436nm) and I-line (365nm) lithography processes from the mid-1970s onwards.The two major promoters of positive photoresist, Hoechst AG and Shipley, saw the former merge with a French company in 1999 and eventually be absorbed into the well-known pharmaceutical company Sanofi in 2004. The latter was first acquired by DuPont, and then after the merger of DuPont and Dow in 2015, it was incorporated into Dow Chemical.The industry continues to evolve, and entering the 1980s, deep ultraviolet (DUV) lithography processes began to be gradually implemented. As the wavelengths used in lithography systems decreased, the intensity of the light source became a key bottleneck limiting lithography efficiency. Equipment manufacturers and material manufacturers made breakthroughs in different fields: on one hand, developing higher power ultraviolet light sources, and on the other hand, improving the photosensitivity of photoresists.Due to the weak intensity of deep ultraviolet (DUV) light sources, traditional DNQ-Novolak photoresists had too slow photosensitivity to meet the fast iteration requirements of the semiconductor industry, making the development of a new type of highly sensitive photoresist urgent.In this context, chemically amplified photoresists emerged. Its concept and realization were mainly attributed to IBM.In 1982, IBM first proposed the concept of chemically amplified photoresists. Its core principle is: using a photo acid generator (PAG) as a catalyst, which produces a strong acid after exposure. At this point, an additional baking step is added after the traditional exposure process, which is crucial. By heating, energy is provided to these acid molecules, making them exceptionally active.These active acid molecules begin to catalyze the deprotection reaction of the photoresist resin on a large scale, which removes the protective groups from the resin and generates hydrophilic functional groups (such as carboxylic acid, which is quite familiar; at this point, it is similar to the principle of DNQ-Novolak photoresists), making it soluble in alkaline developing solution.In this catalytic process, one acid molecule can catalyze hundreds or thousands of resin molecules to undergo deprotection reactions without being consumed itself, which is called “chemical amplification.”Chemically amplified photoresists are a key that opened the door to the deep ultraviolet lithography era, directly promoting the semiconductor process from the micron level to the nanometer level, and has been used until the extreme ultraviolet lithography era.From the historical evolution of photoresists outlined above, it can be seen that the theory and prototypes of photoresists originated in Europe, then commercialized and iterated first in the United States, but the ultimate success belongs to Japanese photoresist companies.In October 2018, the South Korean Supreme Court made a final ruling requiring Japanese companies (such as Nippon Steel and Mitsubishi Heavy Industries) to compensate Korean laborers who were forcibly conscripted during World War II. This trigger directly provoked the anger of the Japanese government, which announced export controls on key semiconductor materials to South Korea in July 2019.These controlled materials mainly included three categories: hydrogen fluoride—used for wafer cleaning; photoresist—the core material for the photolithography process; and fluorinated polyimide—the core material for OLED panels, among which photoresist was the most notable.After Japan announced the export controls, South Korean semiconductor companies collectively fell into a state of shock and panic. At that time, the Korean industry estimated that the domestic stock of photoresist was only enough to last 1-2 months.Less than a week after the export controls were implemented, Lee Jae-Yong, the head of Samsung Group, urgently traveled to Japan to personally meet with several photoresist manufacturers to seek a solution.The incident of Japan cutting off photoresist supply to South Korea was like a sudden stress test, clearly revealing the strong position of Japanese photoresist companies in the global semiconductor industry chain.Usually, they hide behind the brilliance of wafer giants like Samsung and TSMC. But when pushed to the forefront, the world felt their power, which could shake the entire industry.After World War II, Japan’s economy was fully rebuilt, and electronics and semiconductors were listed as strategic emerging industries. In the 1950s, after the Korean War, the United States also intended to support Japan economically, aiming to establish a friendly foothold in Asia.During this period, Japanese companies began to introduce transistor technology from the United States, with Toshiba, Hitachi, NEC, and others gradually obtaining technology licenses from the U.S.In the 1960s, Japan’s economy began to take off, leading to the prosperity of the home appliance industry, which stimulated the establishment of the domestic semiconductor upstream and downstream industry chain.The 1980s was the golden age of Japan’s semiconductor industry. At its peak, Japan’s semiconductor output accounted for about 50% of the global market, especially strong in DRAM manufacturing, once suppressing American giants like Intel.With the active promotion of the Japanese government’s “Very Large Scale Integration (VLSI)” plan, a complete vertical ecosystem was established domestically: from silicon wafers, photoresists, masks, lithography machines, to wafer manufacturing, packaging, and testing processes were almost entirely localized.After years of deep cultivation, Japan currently holds an absolute dominant position in the global photoresist market, especially in high-end semiconductor photoresists, with Japanese companies holding over 80% of the market share. The five core companies are: Tokyo Ohka Kogyo, Shin-Etsu Chemical, JSR Corporation, Fujifilm, and Sumitomo Chemical.In the EUV field, the companies capable of mass-producing EUV photoresists are all Japanese: JSR, Shin-Etsu Chemical, and Tokyo Ohka Kogyo.Looking back over nearly 200 years of development, from asphalt to diazo, photoresist has witnessed humanity’s history of taming light, and this story is far from over.If you find this article well-written, I recommend checking out my previous article “The Misunderstood ‘Li Lianying’ in History.”