Photoresist is a light-sensitive chemical coating used to transfer microscopic patterns onto surfaces during manufacturing. It’s the material that makes it possible to print the billions of tiny circuits on a computer chip, etch the traces on a circuit board, or shape the moving parts of a miniature sensor. When light hits photoresist, it changes the material’s chemistry, making selected areas either washable or permanent so that precise patterns can be carved into the surface beneath.
What Photoresist Is Made Of
A typical photoresist has three main ingredients. The first is a polymer resin, which forms the bulk of the coating and gives it structure. The second is a light-sensitive compound, often called a photoacid generator, that reacts when struck by ultraviolet light. When exposed, these molecules release acids that trigger chemical changes in the surrounding resin. The third ingredient is a solvent that keeps everything in liquid form so it can be spread evenly across a surface. Once applied, the solvent evaporates, leaving behind a thin, uniform film ready for exposure.
The exact chemistry varies depending on the application. Modern semiconductor photoresists are known as chemically amplified resists. In these formulations, a single photon of light can generate an acid molecule that goes on to catalyze hundreds of chemical reactions in the resin, dramatically increasing the material’s sensitivity. This amplification effect is what allows chipmakers to print features measured in nanometers.
Positive vs. Negative Photoresist
Photoresists come in two fundamental types, and the difference comes down to what light does to their chemistry.
- Positive photoresist becomes soluble where light hits it. Exposure breaks the polymer chains apart (a process called chain scission), reducing their molecular weight and making those areas dissolve easily in a developer solution. The pattern you expose is the pattern that washes away, leaving behind the unexposed resist as a protective mask.
- Negative photoresist works in reverse. Light causes the polymer chains to cross-link, hardening the exposed areas into a tough, insoluble coating. The unexposed regions wash away instead, so the pattern you expose is the pattern that stays.
Positive resists generally produce sharper, finer features, which is why they dominate in semiconductor manufacturing. Negative resists tend to be tougher and more chemically resistant, making them useful for applications where the coating needs to withstand harsh etching or plating steps.
How Photoresist Is Used in Manufacturing
The full process of using photoresist is called photolithography, and it follows a consistent sequence of steps whether you’re making a processor, a circuit board, or a micro-sensor.
First, the surface (usually a silicon wafer) is dehydrated by baking to remove all moisture, ensuring the photoresist will stick properly. Next, liquid photoresist is dispensed onto the center of the wafer and the wafer is spun at high speed. The spinning spreads the resist into an extremely thin, uniform layer, typically less than a micrometer thick for advanced chips.
The coated wafer is then aligned beneath a photomask, a glass plate with the desired circuit pattern printed on it in chrome. Ultraviolet light shines through the mask, projecting the pattern onto the photoresist below. Only the areas where the mask is transparent receive light.
After exposure, the wafer goes into a chemical developer bath. For positive resists, this washes away the exposed areas. For negative resists, it washes away the unexposed areas. What remains is a precise stencil of hardened resist protecting certain parts of the wafer surface. The wafer is then inspected under a microscope to confirm the patterns are clean and well defined, and a final hard bake further strengthens the remaining resist for the etching or deposition steps that follow.
Why Light Wavelength Matters
The smallest feature you can print with photoresist is limited by the wavelength of light used to expose it, just as the sharpness of a projected image depends on the fineness of the light source. Over decades, the semiconductor industry has pushed to shorter and shorter wavelengths to shrink transistor sizes.
Early lithography used visible and near-ultraviolet light. The industry then moved to deep ultraviolet (DUV) light at 193 nanometers, which remains the workhorse wavelength for most chip production today. The latest generation uses extreme ultraviolet (EUV) light at just 13.5 nanometers, enabling features smaller than 30 nanometers. Researchers are already exploring beyond-EUV wavelengths around 6.5 nanometers for even finer patterning.
Each jump to a shorter wavelength requires entirely new photoresist chemistry. EUV light is so energetic that it interacts with materials differently than DUV light does, and the resists must be redesigned to respond efficiently at these wavelengths while still producing clean, sharp edges. One persistent challenge is that shorter wavelengths deliver fewer photons per unit area, which introduces statistical noise that can roughen the edges of printed lines.
Where Photoresist Is Used
Semiconductor chips are the highest-profile application, but photoresist is essential across a range of industries. Printed circuit boards rely on photoresist to define the copper traces that connect components. Micro-electromechanical systems (MEMS), the tiny accelerometers and pressure sensors in your phone, are shaped using photoresist-based lithography. Display panels, LED arrays, and even some biomedical devices use photoresist patterning at various scales.
The requirements differ by industry. Chipmakers need resists that can resolve features below 10 nanometers. Circuit board manufacturers work at scales thousands of times larger but need resists that hold up against aggressive copper etching baths. MEMS fabrication often demands thick resist layers that can form tall, three-dimensional structures with steep sidewalls.
Environmental and Safety Concerns
Photoresist chemicals pose real handling and disposal challenges. The solvents used in resist formulations are flammable and toxic, requiring careful ventilation and protective equipment in fabrication facilities. After exposure and development, the spent developer solution contains chemicals that must be treated before disposal.
One growing concern involves fluorinated compounds. Some advanced photoresists, particularly those designed for EUV lithography, incorporate fluorinated polymers to improve performance. These fluorine-containing materials fall into the broader category of per- and polyfluoroalkyl substances (PFAS), sometimes called “forever chemicals” because they resist breaking down in the environment. The EPA has proposed listing nine specific PFAS compounds as hazardous constituents under federal waste regulations, citing links to reproductive harm, developmental effects, immune suppression, and increased cancer risk. This regulatory pressure is pushing resist manufacturers to develop formulations with reduced or eliminated fluorine content, though the transition is complicated by the performance advantages fluorinated resists provide at cutting-edge wavelengths.