We are all made of star stuff, as astronomer Carl Sagan was fond of reminding us. Supernova explosions, the catastrophic self-destruction of certain types of worn-out stars, are intimately tied to life on Earth because they are the birthplaces of heavy elements across the universe. Most of the iron in our blood and the sulfur in our amino acids originated in stars that detonated billions of years ago. But we have encountered another, quite surprising connection between supernovas and the human world—specifically, a connection to the technology needed to make computer chips for the latest smartphones and other electronic devices.
That connection emerged several years ago in a series of conversations between myself,
Jayson Stewart, and my grandfather Rudolf Schultz. My grandfather was an avid amateur sky gazer who kept a large reflector telescope in the foyer of his home, right by the entrance, ready for rapid deployment. When I was in high school, he handed me a copy of Stephen Hawking’s A Brief History of Time (Bantam Books, 1988) and guided me toward a lifelong love of physics. More recently, my grandfather’s astronomical perspective also proved serendipitously useful in my career, as I explained to him during one of our observation sessions at his home in the foothills of Tucson.
A double laser zap strikes a droplet of tin in ASML’s lithography machine. The first zap flattens the drop into a disk; the second vaporizes it into a ball of hot, ultraviolet-emitting plasma. ASML
I was updating my grandfather about the work I was doing in my lab at
ASML, a Netherlands-based company that develops and builds equipment for manufacturing semiconductor chips. At the time, about a decade ago, I was helping to refine a system for fabricating chips using extreme ultraviolet( EUV) light. Although it is critical to making the most advanced microchips today, EUV lithography was then a challenging technology still in development. To produce EUV light, we would focus an intense laser pulse onto 30-micrometer-wide droplets of tin flying through a chamber filled with low-density hydrogen. Energy from the laser transformed the droplets into balls of plasma that are 40 times as hot as the surface of the sun, causing the tin to emit intense ultraviolet radiation. As a by-product, the plasma balls generated shock waves that traveled through the surrounding hydrogen. Unfortunately, the explosions also released sprays of tin debris, which were proving extremely difficult to manage.
Recalling my astronomy lessons with my grandfather, I realized that many aspects of this process have intriguing similarities to what happens during a supernova: a sudden explosion, an expanding cloud of plasma debris, and a shock wave that slams into a thin hydrogen environment. (Interstellar material consists mostly of hydrogen.) To refine our EUV setup, we would record the evolution of the shock wave from our plasma balls, much as astronomers study the remains of supernovas to deduce the properties of the stellar explosion that created them. We even used some of the same equipment, such as a filter tuned to the characteristic deep-red emission of energized hydrogen atoms, called a Hydrogen-alpha, or H-alpha, filter. Despite the fact that a supernova has 1045 times as much energy as our tin blasts, the same math describes the evolution of both types of explosions. The close physical analogy between tin-plasma shocks and supernova shocks has turned out to be key to figuring out how to deal with our vexing tin-debris problem.
Seen through telescopic eyes, the night sky is dotted with the glowing remains of exploded stars. My grandfather was tickled by the connection between these ancient, distant celestial objects and the modern equipment used to make the most advanced semiconductor chips in the world. He felt that many other amateur sky gazers like himself would love to read about this story. I told him I would write it up if he would be my coauthor—and he is.
Sadly, my grandfather is not here to see our article completed. But he did live to see these astrophysical parallels lead to important practical consequences: They helped my group at ASML produce a bright, reliable EUV light source, leading to a
major advance in commercial chipmaking.
EUV and Moore’s Law
My journey into the world of EUV mini-supernovas started in 2012, when I was completing a stint as a postdoctoral research scientist at
Los Alamos National Laboratory and looking for my first job outside of academia. A friend got me interested in the possibilities of working in the semiconductor industry, where manufacturers are engaged in a constant, high-stakes competition to build smaller, faster circuits. I learned that the lithography process used to create features on computer chips was at a crisis point, one that offered intriguing engineering challenges.
In lithography, light is used to imprint an intricate pattern onto a prepared silicon substrate. This process is repeated many times in a series of etching, doping, and deposition steps to create up to one hundred layers; the patterns in those layers end up defining the circuitry of a computer chip. The size of the features that can be transferred onto that silicon substrate is determined by the imaging system and by the wavelength of light. The shorter the wavelength and more energetic the light, the smaller the features. The ultraviolet wavelengths in use at the time were too long and crude for the next generation of chips. Lithography technology, and potentially the nearly trillion-dollar electronics industry, would stagnate unless we could create a powerful source of shorter-wavelength, EUV light.
At the time, the available EUV light sources were too feeble by about a factor of 10. The task of achieving such a huge power increase was so daunting that I debated with my family about the wisdom of starting a career in EUV lithography. Plenty of pundits suggested that the technology could never be commercialized. Despite my trepidation, I was won over by Daniel Brown, then ASML’s vice president of technology development, who saw EUV as the best way to achieve the next big jump in chip performance. (Daniel, a coauthor of this article, retired from the company at the end of 2024.)
Amazingly, the Taylor-von Neumann-Sedov formula describes atomic–bomb shocks with radii of hundreds of meters, supernova shocks that stretch across light years, and tin-plasma shocks just millimeters wide.
For decades, manufacturers had managed to squeeze more and more transistors onto an integrated circuit, going from about 2,000 transistors in 1971 to 200 billion in 2024. Engineers kept Moore’s Law—the doubling of transistor count every couple of years—alive for more than five decades by incrementally reducing the wavelength of light and expanding the numerical aperture of the imaging system used in lithography.
Lithography systems in the 1980s used mercury lamps that radiated at wavelengths of 436 nanometers (violet light) and eventually 365 nm (near-ultraviolet). To reduce the feature size of transistors further, people invented high-power lasers that could produce ultraviolet beams at shorter, 248-nm and 193-nm wavelengths. Then the move to ever-shorter wavelengths hit a wall, because almost all known lens materials absorb light with wavelengths of less than about 150 nm.
For a little while, lithographers managed to keep making progress using a clever trick: They
put water between the lens and the silicon wafer to improve the focusing power of the imaging system. But eventually, the scaling process stagnated and engineers were forced to switch to shorter wavelengths. That switch, in turn, required replacing lenses with mirrors, which came with a penalty. Mirrors could not achieve the same focusing precision as the previous lens-plus-water combination. To make meaningful progress, we needed to drastically reduce the wavelength of the light to around 13.5 nm, or about one-thirtieth the wavelength of the shortest visible violet light that your eye can see.
To get there, we’d need something insanely hot. The wavelength of light emitted by an incandescent source is determined by its temperature. The surface of the sun, which has a temperature of 6,000 °C, radiates most strongly in the visible spectrum. Getting to EUV light with a wavelength of 13.5 nm requires a source with an extremely high temperature, around 200,000 °C.
Tin droplets fall through ASML’s lithography machine. Laser beams strike the passing droplets 50,000 times a second, causing them to glow and creating a continuous extreme ultraviolet light source. Tin debris is swept away by a high-speed flow of hydrogen. ASML
At ASML, we settled on a hot, energetic tin plasma as the best way to create an EUV “lightbulb.” Because of the particular way their electrons are arranged, highly excited tin ions radiate much of their light in a narrow band right around the industry’s desired 13.5-nm wavelength.
The big question we faced was how to create such a tin plasma reliably. The lithography process in chip manufacturing requires a specific, highly consistent EUV radiation dose to expose the photoresist, the light-sensitive material used to create circuit patterns on the wafer. So the light source had to deliver accurate amounts of energy. Equally important, it had to do so continuously for long periods of time, with no costly pauses for repair or maintenance.
We designed a
Rube Goldberg–like system in which a molten droplet of tin is targeted by two laser beams. The first turns the droplet into a pancake-shaped disk. The second laser hits the tin with a short, energetic laser pulse that converts it into a high-temperature plasma. A nearly hemispherical, multilayer mirror then collects EUV light from the plasma and projects it into the lithographic scanner, a bus-size tool that uses the light to project patterns onto the silicon wafer.
The modern chipmaking process begins with an extreme ultraviolet (EUV) light source. The EUV light is directed by an elaborate series of mirrors onto the surface of a moving wafer, where it creates the desired pattern of imprinted circuits. ASML
Sustaining an EUV light source intense enough for lithography requires a primary laser with a power of several tens of kilowatts, zapping about 50,000 droplets of tin every second. In less than one ten-millionth of a second, each laser pulse transforms the tin from a 30-micrometer-wide droplet into a millimeter-wide plasma explosion with tens of thousands of times its original volume.
Mark Phillips, the director of lithography and hardware solutions at Intel, described the EUV lithography machine we were helping to develop as “the most technically advanced tool of any kind that’s ever been made.”
At 50,000 droplets per second, operating under heavy use, each of our lithography machines has the potential to generate nearly 1 trillion pulses per year, totaling many liters of molten tin. Through all of that, a single nanometer of tin debris coating the collector optic would degrade the EUV transmission to unacceptable levels and put the machine out of commission. As we say in the industry, it wasn’t enough to make the power; we had to
survive the power.
Hydrogen in EUV and in Space
A continuous flush of low-density hydrogen gas protects the mirror and surrounding vessel from the spray of vaporized tin ejecta. That debris has an initial velocity of tens of kilometers per second, much faster than the speed of sound in hydrogen. When the supersonic tin hits the hydrogen gas, it therefore produces an outward-spreading shock wave—the one that is closely analogous to what happens when a supernova explosion expands into the tenuous hydrogen that fills interstellar space.
The low-density hydrogen gas is also on the move, though, flowing through the machine at hundreds of kilometers per hour. The gas slows, cools, and flushes out the energetic tin debris as it goes. To determine how much hydrogen we needed to sweep the tin away and to keep the gas from overheating, first we had to figure out the total energy released by the laser-produced plasmas. And figuring out that amount was not a trivial task.
My colleagues and I at ASML found an effective way to measure the energy of our tin explosions, not by studying the plasma directly, but by observing the response of the hydrogen gas. In hindsight the idea seems clear, but in the moment, there was a lot of fumbling around. When I was taking images of the tin plasma, I kept observing a much larger, red glowing orb surrounding it. It seemed likely that the plasma blast was inducing H-alpha emission from the hydrogen. But the observations left us with many unknowns: Why are the orbs that specific size (millimeters in diameter), how do they evolve, and, most important, how can we study the glow to measure the energy deposited into the gas?
The shock wave produced by a laser-heated tin droplet in a thin hydrogen atmosphere is similar enough to a supernova blast that they can both be described by the same math. The whole sequence takes less than a millionth of a second. ASML
I examined the red orbs using a
Teledyne Princeton Instruments Pi-Max 4, an ultrafast, intensified CCD camera that can perform rapid exposure times on the order of nanoseconds. I paired it with a long-distance microscope lens, to collect the glow from those red orbs, and an Orion 2-inch extra-narrowband H-alpha bandpass filter that I purchased from an astrophotography website. The images I captured with this rig were striking. Every plasma event was sending out a spherical shock front that expanded in a steady way.
By chance, months earlier, I had attended a seminar that mentioned blast waves—shock waves produced by a point-source explosion. That seminar convinced me that our observations could give me the energy measurement I was looking for. In my hunt to understand how blast waves evolve, I learned that astronomers had run into the same measurement problem when attempting to determine the initial energy release that had produced an observed supernova remnant. And I knew that I also had the perfect topic for the next of my ongoing science talks with my grandfather.
The Taylor-von Neumann-Sedov formula was developed in the 1940s to calculate the yield of atomic bombs, but it also describes the evolution of plasma shock waves in our EUV lithography system and in distant supernovas. It relates the shock wave’s radius (R) over time to the energy released (E), gas density (ρ), and a gas-dependent parameter (C).
To get an answer, astronomers turned to equations that were discovered in the 1940s, when scientists were seeking ways to analyze the destructive capacity of newly developed
atomic weapons. One expression of those equations, called the Taylor-von Neumann-Sedov formula, describes the radius of the shock as a function of time. It provides a simple, direct relationship between the radius of the shock and the total energy.
In 1949, British physicist
Geoffrey Taylor used his newly derived formulation of blast waves to determine and publish the (then-classified) energy yield of the first atomic-bomb detonations. Taylor’s success, which reportedly upset the United States government, demonstrated the power of his analysis. Amazingly, the Taylor-von Neumann-Sedov formula describes atomic-bomb shocks with radii of hundreds of meters, supernova shocks that stretch across light years, and tin-plasma shocks just millimeters wide. They all represent the same basic physical situation: a compact, freestanding body releasing energy against minimal resistance, expanding rapidly into a gaseous surrounding.
Early atomic explosions, such as this test at the Trinity Site on 16 July 1945, inspired scientists to develop new math to calculate the amount of energy released. U.S. Department of Energy
Applying the Taylor-von Neumann-Sedov formula to the H-alpha images we recorded in the ASML light source resulted in a satisfying agreement between our calculated energies and the amounts we had roughly estimated by other means. We also encountered some discrepancies between theory and practice, however. In our EUV sources, we observed that the H-alpha emission is not always perfectly symmetric, which may indicate that our laser-produced plasmas do not quite match the simplifying “point-source” assumption. We also tried varying a number of different parameters to learn more about the blasts (a type of experiment that is obviously not possible for supernovas). For instance, we mapped blast-wave trajectories as a function of ambient pressure, droplet size, laser energy, and target shape.
Our results helped us to refine our models and to determine the best way to tailor the hydrogen environment in our machines to enable a clean, stable EUV source for chip fabrication.
Ad Astra per Aspera
The connection between supernovas and laser-produced plasmas is just one example of a long history of advances in physics and engineering that were inspired by astronomy. For centuries, researchers have designed laboratory experiments and measurement techniques to re-create what was observed in the sky. The modern description of the atom can trace its roots to the invention of the prism and the spreading of the solar spectrum into its composite colors, which led to the identification of discrete energy levels in an atom and, finally, the development of quantum mechanics. Without quantum mechanics, many modern electronics technologies would not be possible.
Barnard’s Loop [left], in the constellation Orion, is the remnant of an ancient supernova. It glows in Hydrogen-alpha light, just like the shock waves produced by tin-plasma explosions in ASML’s light source. Daniel Brown
The spread of ideas has gone the other way as well. As the rules of atomic physics and the absorption lines of gases were characterized in lab experiments, astronomers used spectroscopic observations to determine the composition of the sun, to deduce the life cycles of stars, and to measure the dynamics of galaxies.
I find it fascinating that the laser-produced plasmas we use in our EUV light source especially resemble one particular variety of supernova, known as Type Ia. This kind of supernova is thought to occur when a white dwarf star pulls material from a neighboring companion star until it reaches a critical mass and implodes, resulting in a violent self-destruction. Type Ia supernovas explode in a highly consistent way, making them valuable “standard candles” with predictable intrinsic luminosities: Comparing their apparent brightness to their true, intrinsic luminosity makes it possible to measure their distances from us accurately across billions of light years. These supernovas are being used to study the expansion of the universe, and they have led to the startling discovery that the expansion of the cosmos is accelerating.
In our EUV sources, we likewise aim to have all of our explosions identical, so that they serve as a “standard candle” for the EUV scanner. Our aims are decidedly more earthly than cosmic in scale, but our ambitions are grand all the same.
This article was updated on 05 March 2025.
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