Semiconductor Fabrication: From Sand to Chip · Module 4
Lithography
Lithography prints the patterns that define every part of a chip. More than any other step, it sets how small a transistor can be.
What you will learn
- The sequence of steps that prints a pattern on a wafer
- The difference between positive and negative photoresist
- What limits the smallest feature that light can print
- How immersion, multiple patterning, and EUV pushed that limit
In plain language
Lithography works much like photographic printing. The wafer is coated with a light-sensitive liquid called photoresist. Light then shines through a mask that carries the pattern. Where the light lands, the resist changes chemically, and a developer solution washes those areas away. What remains is a stencil of resist on the wafer.
The next step, such as etching or doping, acts only where the resist is gone. Afterward the resist is removed and the cycle starts again for the next layer. An advanced chip needs this cycle dozens of times, and each new pattern must line up with the ones beneath it to within a few nanometers.
ANNLab, University of Arizona
Figure 1. The lithography sequence with a positive resist, followed by etching of the film and removal of the resist.
Core lesson
The lithography sequence
The wafer is first cleaned and treated with an adhesion promoter so the resist will stick. Resist is dropped onto the center, and the wafer is spun at a few thousand revolutions per minute. Spinning spreads the resist into a film whose thickness depends on the spin speed and on how viscous the resist is. A short bake on a hot plate drives off the solvent.
The wafer is then aligned to the mask and exposed. After a second bake, it is developed in a water-based solution that dissolves part of the resist. A final bake hardens what remains. The pattern is inspected at this point because lithography can still be undone. If the pattern is wrong, the resist is stripped and the wafer is coated again. Etching, which comes next, is permanent.
Positive and negative resist
In a positive resist, the exposed areas dissolve in the developer, so the resist pattern matches the opaque parts of the mask. In a negative resist, the exposed areas harden and stay while the rest dissolves. Most advanced lithography uses positive resists in which each absorbed photon starts a chain of chemical reactions during the bake after exposure. This amplification lets a small dose of light pattern the resist.
Exposure tools
The simplest tool presses the mask against the wafer and exposes the whole wafer at once. Contact printing is common in research and teaching cleanrooms, but contact wears the mask and limits resolution to about a micrometer. Production fabs use projection. A lens system forms a reduced image of the mask on the wafer, usually four times smaller. The tool, called a scanner, exposes one small field at a time and steps across the wafer until it is covered.
Research labs also write patterns directly with no mask. Electron-beam lithography draws with a focused beam of electrons and reaches a few nanometers, though it is slow. Laser direct writing draws micrometer-scale patterns quickly and works well for trying new designs.
What limits resolution
Light bends when it passes the edges of the mask pattern, an effect called diffraction. This blurs the image. The smallest feature a projection tool can print is proportional to the wavelength of the light. It also shrinks as the numerical aperture of the lens grows, which is a measure of how wide a cone of light the lens collects.
The industry has therefore moved to shorter wavelengths, from the 436 nm and 365 nm lines of the mercury lamp to lasers at 248 nm and then 193 nm. Filling the gap between the lens and the wafer with water, called immersion lithography, raised the numerical aperture to 1.35 and extended 193 nm light further.
Multiple patterning and EUV
When features became much smaller than 193 nm light could print in one exposure, fabs split one layer into two or more exposures. They also used a method in which thin films deposited on the sides of printed lines become the pattern, which doubles the number of lines. These methods work, but they add many steps.
Extreme ultraviolet lithography, or EUV, uses light with a wavelength of 13.5 nm. This light is absorbed by almost everything, including air and glass, so the whole optical path is in vacuum and uses mirrors in place of lenses. Even the mask is a mirror. The light is produced by firing a powerful laser at tiny droplets of molten tin. EUV entered volume production around 2019, and a newer generation of tools with a larger numerical aperture of 0.55 is now being introduced.
Going deeper
These sections add the equations behind the core lesson, each with a worked example. They assume first-year algebra and can be skipped without losing the thread of the course.
The smallest half-pitch, which is half the distance between repeating lines, is given by the Rayleigh equation.
R = k1 λ / NA
Here λ is the wavelength, NA is the numerical aperture, and k1 is a process factor that cannot go below 0.25 for a single exposure.
Worked example
For 193 nm immersion with NA = 1.35 and k1 = 0.28, R = 0.28 × 193 / 1.35 = 40 nm. For EUV with λ = 13.5 nm, NA = 0.33, and k1 = 0.32, R = 0.32 × 13.5 / 0.33 = 13 nm. One EUV exposure prints what would take several immersion exposures.
The image is sharp only within a small range of height, called the depth of focus.
DOF = k2 λ / NA2
Worked example
For λ = 193 nm, NA = 0.93, and k2 = 0.5, DOF = 0.5 × 193 / 0.865 = 112 nm. The wafer surface and the resist must stay within about a tenth of a micrometer of the focal plane across the whole exposure field. This is why wafers are polished flat before critical lithography steps, as Module 9 describes. A larger NA improves resolution but reduces the depth of focus.
For a given resist, the film thickness is roughly proportional to one over the square root of the spin speed.
Worked example
A resist gives a film of 1.4 µm at 3,000 revolutions per minute. At 6,000 revolutions per minute the thickness is 1.4 / √2 = 1.0 µm. To change the thickness by a large factor, fabs choose a resist with a different viscosity.
Check your understanding
Try each question before opening the answer. The last question uses the Going deeper material.
They become soluble and are washed away by the developer. The unexposed resist stays on the wafer.
Photoresist reacts to blue and ultraviolet light. Yellow light contains neither, so the resist is not exposed by the room lighting.
Use light with a shorter wavelength, or use a lens with a larger numerical aperture. Improving the process to lower the k1 factor is a third way.
Light at 13.5 nm is absorbed by air and by glass. Lenses would absorb it, so the tool uses reflecting mirrors, and the path is kept in vacuum so the light reaches the wafer.
R = 0.40 × 248 / 0.80 = 124 nm.
Key terms
- Photoresist. A light-sensitive coating that becomes more or less soluble where it is exposed
- Mask. A plate that carries the pattern for one layer, also called a reticle
- Developer. The solution that dissolves the soluble part of the resist after exposure
- Positive and negative resist. Resists in which the exposed areas dissolve (positive) or remain (negative)
- Diffraction. The bending of light at edges, which blurs small features
- Numerical aperture. A measure of how wide a cone of light a lens collects
- Scanner. A projection tool that exposes one field at a time and steps across the wafer
- Immersion lithography. Lithography with water between the lens and the wafer to raise the numerical aperture
- EUV lithography. Lithography with 13.5 nm light, using mirrors in vacuum
- Multiple patterning. Building one layer from two or more exposures or from deposited sidewall films
- Overlay. How accurately a new pattern lines up with the patterns beneath it
In the fab
Lithography engineers and technicians run the most expensive tools in the fab. A single EUV scanner costs well over 100 million dollars, so keeping it running and in focus is a central job.
Further reading
J. D. Plummer and P. B. Griffin, Integrated Circuit Fabrication: Science and Technology, Cambridge University Press. The authors offer a prepublication version that is free to read on the Stanford book site (opens in a new tab).
To practice the steps yourself, try the Digital Twin Photolithography Training on our Training page.
Previous: Module 3Course homeNext: Module 5, Oxidation and Dielectrics
Created by Prof. Zafer Mutlu, ANNLab, University of Arizona.