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Immersion lithography

Immersion lithography is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Immersion lithography rather than just read about it. In short: Immersion lithography is a technique used in semiconductor manufacturing to enhance the resolution and accuracy of the photolithographic process. It involves using a liquid medium, typically water, between the lens and the wafer during exposure.

Immersion lithography — main illustration
Immersion lithography — illustration

Key takeaways

  • Immersion lithography belongs to science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Immersion lithography to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Immersion lithography from memory before moving on to harder problems.

Reference excerpt

Immersion lithography is a technique used in semiconductor manufacturing to enhance the resolution and accuracy of the photolithographic process. It involves using a liquid medium, typically water, between the lens and the wafer during exposure. By using a liquid with a higher refractive index than air, immersion lithography allows for smaller features to be created on the wafer. Immersion lithography replaces the usual air gap between the final lens and the wafer surface with a liquid medium that has a refractive index greater than one. The angular resolution is increased by a factor equal to the refractive index of the liquid. Current immersion lithography tools use highly purified water for this liquid, achieving feature sizes below 45 nanometers.

Background The ability to resolve features in optical lithography is directly related to the numerical aperture of the imaging equipment, the numerical aperture being the sine of the maximum refraction angle multiplied by the refractive index of the medium through which the light travels. The lenses in the highest resolution "dry" photolithography scanners focus light in a cone whose boundary is nearly parallel to the wafer surface. As it is impossible to increase resolution by further refraction, additional resolution is obtained by inserting an immersion medium with a higher index of refraction between the lens and the wafer. The blurriness is reduced by a factor equal to the refractive index of the medium. For example, for water immersion using ultraviolet light at 193 nm wavelength, the index of refraction is 1.44. The resolution enhancement from immersion lithography is about 30–40% depending on materials used. However, the depth of focus, or tolerance in wafer topography flatness, is improved compared to the corresponding "dry" tool at the same resolution. The idea for immersion lithography was patented in 1984 by Takanashi et al. It was also proposed by Taiwanese engineer Burn J. Lin and realized in the 1980s. In 2004, IBM's director of silicon technology, Ghavam Shahidi, announced that IBM planned to commercialize lithography based on light filtered through water.

Defects Defect concerns, e.g., water left behind (watermarks) and loss of resist-water adhesion (air gap or bubbles), have led to considerations of using a topcoat layer directly on top of the photoresist. This topcoat would serve as a barrier for chemical diffusion between the liquid medium and the photoresist. In addition, the interface between the liquid and the topcoat would be optimized for watermark reduction. At the same time, defects from topcoat use should be avoided. As of 2005, Topcoats had been tuned for use as antireflection coatings, especially for hyper-NA (NA>1) cases. By 2008, defect counts on wafers printed by immersion lithography had reached zero level capability.

Polarization impacts As of 2000, Polarization effects due to high angles of interference in the photoresist were considered as features approach 40 nm. Hence, illumination sources generally need to be azimuthally polarized to match the pole illumination for ideal line-space imaging.

Throughput

As of 1996, this was achieved through higher stage speeds, which in turn, as of 2013 were allowed by higher power ArF laser pulse sources. Specifically, the throughput is directly proportional to stage speed V, which is related to dose D and rectangular slit width S and slit intensity Iss (which is directly related to pulse power) by V=Iss*S/D. The slit height is the same as the field height. The slit width S, in turn, is limited by the number of pulses to make the dose (n), divided by the frequency of the laser pulses (f), at the maximum scan speed Vmax by S=Vmax*n/f. At a fixed frequency f and pulse number n, the slit width will be proportional to the maximum stage speed. Hence, throughput at a given dose is improved by increasing maximum stage speed as well as increasing pulse power. In 2015 ASML reported that their TWINSCAN NXT:1980Di immersion lithography platform archieved throughputs of up to 275 WPH, targeted for high volume manufacturing.

Multiple patterning

The resolution limit for a 1.35 NA immersion tool operating at 193 nm wavelength is 36 nm. Going beyond this limit to sub-20nm nodes requires multiple patterning. In 2016 Samsung announced successful tests of a 10nm SoC using triple patterning, slated for mass production in 2017.

See also Oil immersion Water immersion objective

References

Illustrations

Immersion lithography: In immersion lithography, light travels down through a system of lenses and then a pool of water before reaching the photoresist on top of the wafer.
In immersion lithography, light travels down through a system of lenses and then a pool of water before reaching the photoresist on top of the wafer.
Immersion lithography: Throughput of immersion lithography tools vs. dose. The throughput vs. dose is compared to for different pulse powers at the same slit width.
Throughput of immersion lithography tools vs. dose. The throughput vs. dose is compared to for different pulse powers at the same slit width.
Immersion lithography: Double patterning by pitch splitting. Double patterning by pitch splitting involves assigning adjacent features to different masks, indicated by different colors.
Double patterning by pitch splitting. Double patterning by pitch splitting involves assigning adjacent features to different masks, indicated by different colors.
Immersion lithography: Triple patterning by pitch splitting. Triple patterning by pitch splitting involves assigning adjacent features to 3 different masks, using three colors.
Triple patterning by pitch splitting. Triple patterning by pitch splitting involves assigning adjacent features to 3 different masks, using three colors.

Worked examples

Example 1 — a first encounter with Immersion lithography

Start with the simplest possible case. Write down what Immersion lithography claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Immersion lithography before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Immersion lithography ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Immersion lithography

In research
Immersion lithography appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Immersion lithography in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Immersion lithography is common in secondary-school and first-year university syllabi. It links to neighbouring topics Lithography (microfabrication), Taiwanese inventions, so understanding it makes those chapters shorter.
In everyday life
Look for Immersion lithography outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Immersion lithography in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Immersion lithography means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Immersion lithography out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Immersion lithography in simple terms?

Immersion lithography is a technique used in semiconductor manufacturing to enhance the resolution and accuracy of the photolithographic process. It involves using a liquid medium, typically water, between the lens and the wafer during exposure.

Why does Immersion lithography matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Immersion lithography?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Immersion lithography.

Tags

  • Lithography (microfabrication)
  • Taiwanese inventions

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