[Photoresist Series Part 7] Sensitivity, Resolution, and LER: The Difficult Triangle

What kind of material makes for an excellent photoresist?

A material that reacts with a low exposure dose, can form extremely fine patterns, and has smooth line edges with few defects.

Ideally, we would like to satisfy all of these simultaneously.

However, in actual resist development, improving one performance metric can sometimes cause another to deteriorate.

At the center of this are

• Sensitivity

• Resolution

• Line Edge Roughness (LER)

these three.

These are generally referred to as the RLS trade-off, and they have become a core challenge, particularly in EUV resist development.

In this installment, we will delve into why this triangle relationship exists, what material manufacturers are adjusting, and what will change in the High-NA era.

What is sensitivity?

Sensitivity is a performance metric indicating how little exposure dose is required to form the necessary pattern.

Generally, the lower the exposure dose required to obtain a specified CD, the higher the sensitivity.

High sensitivity offers significant advantages.

• Exposure time can be shortened

• The number of wafers processed can be increased

• The productivity of expensive exposure equipment can be improved

• Requirements for light source output can be reduced

EUV equipment in particular is extremely expensive, and the exposure dose is directly linked to throughput.

Therefore, material manufacturers are required to provide resists that react sufficiently at low doses.

However, as the exposure dose is reduced, the number of photons also decreases, leading to greater statistical variation.

While increasing sensitivity is advantageous for productivity, it can be disadvantageous for stochastic defects and LER.

What is resolution?

Resolution is the performance capability of how small a line, space, or hole can be formed while remaining separated.

It is not just about seeing the lines, but also ensuring that:

• adjacent patterns do not connect

• holes open reliably

• the shape remains close to the design dimensions

• there is sufficient process margin

are necessary.

Resist resolution is not determined solely by the optical performance of the exposure equipment.

• polymer molecular size

• acid diffusion distance

• development contrast

• Film thickness

• Underlayer film

• Pattern collapse

• Etch resistance

and others also have an impact.

Even if the exposure image has high resolution, if the reaction within the resist spreads too much, the final pattern will be blurred.

In other words, the resist is required to have the ability to faithfully convert an optical image into a chemical image.

What is LER?

LER stands for Line Edge Roughness, which indicates how much the edge of a line fluctuates from an ideal straight line.

When the fluctuations of the edges on both sides appear as variations in line width, it is called LWR (Line Width Roughness).

If LER or LWR is large, the local line width will vary.

The impact is not merely a matter of appearance.

• Variations in transistor characteristics

• Fluctuations in wiring resistance

• Increase in leakage current

• Occurrence of opens and shorts

• Non-uniformity in device performance

can lead to these issues.

As line widths shrink due to miniaturization, the same 1 nm of roughness accounts for a larger proportion of the line width.

Therefore, the importance of LER increases at more advanced nodes.

Why can't all three be improved simultaneously?

Increasing sensitivity leads to more photon noise.

When reacting with a lower exposure dose, the number of photons reaching a local area decreases.

The fewer the number of photons, the relatively larger the statistical variation from place to place becomes.

As a result, fluctuations in the amount of reaction lead to LER and stochastic defects.

Increasing acid diffusion improves sensitivity but decreases resolution.

In chemically amplified resists, the acid generated during exposure moves during PEB and promotes numerous deprotection reactions.

If the acid moves widely, sensitivity increases because a large reaction area can be created from fewer photons.

On the other hand, if the acid moves beyond the boundary between exposed and unexposed areas, the pattern edges become blurred, and resolution and LER deteriorate.

Suppressing acid diffusion improves resolution but decreases sensitivity.

Increasing the quencher or reducing the mobility of the acid can sharpen the reaction boundary.

However, because the reaction range becomes narrower, it becomes necessary to increase the exposure dose to obtain the same solubility change.

In this way, these three performance characteristics are linked through common molecular reactions.

Is the "love triangle" a fixed law?

The RLS trade-off is an important concept, but it should not be thought of as a "fixed limit that can never be improved."

It is possible to improve the overall balance of these three performance metrics through new material design and process technology.

However, it means that if you only adjust one knob while using conventional materials, it is easy for the burden to shift to another performance metric.

The goal of material development is not just to find a compromise within the triangle.

is the goal.

Polymer molecular size and LER

Although resist patterns appear to be continuous objects, they are actually made of molecules of finite size.

As pattern dimensions approach molecular size, molecular granularity can no longer be ignored.

In polymers with broad molecular weight distributions or films with local density variations, development boundaries tend to become non-uniform.

Therefore,

• narrowing the molecular weight distribution

• using more uniform small-molecule materials

• bonding PAG to the polymer

• moving toward single-component types

are designs that are being considered.

However, making molecules smaller is not always better.

It is necessary to balance this with film-forming properties, glass transition temperature, heat resistance, etching resistance, and outgassing.

The balance between PAG and quencher

Increasing the PAG improves sensitivity by raising the probability of acid generation after light absorption.

However, if the PAG is locally uneven, the variation in reaction volume increases.

Quenchers suppress excessive acid diffusion, but if there are too many, they deactivate too much acid, reducing sensitivity.

Therefore, what is important is not just simple concentration.

• PAG distribution

• Quencher distribution

• Acid strength

• Acid diffusion distance

• Reaction rate

• Interaction with polymers

must be homogenized at the nanometer scale.

In IBM's research as well, for EUV PAGs, designing to simultaneously improve resolution, roughness, and sensitivity while reducing stochastic defects remains a continuous research challenge.

Development contrast also influences the triangle relationship

The greater the difference in dissolution rate between the exposed and unexposed areas, the clearer the development boundary becomes.

High development contrast is advantageous for resolution and shape fidelity.

However, if the dissolution reaction is too rapid, local reaction differences can appear as pattern defects.

Furthermore, developer, time, temperature, agitation, and rinse conditions also affect LER and pattern collapse.

RLS is not determined solely by exposure and resist composition, but is the performance of the entire process, including development.

A new trade-off: thinning the film

In High-NA EUV, the depth of focus becomes shallower, making it necessary to thin the resist film.

Thin resists have their advantages.

• Adaptation to depth of focus

• Reduction in aspect ratio

• Suppression of pattern collapse

On the other hand, there are also problems.

• Difficulty in resisting etching

• Decrease in the number of molecules present in the film

• Increase in local statistical variation

• Difficulty in pattern transfer to the underlying layer

For this reason, with materials for High-NA, it is essential to optimize not only the RLS of the resist itself but also the underlying layer, hard mask, and etching in collaboration.

Can metal oxide resists break the triangle?

Metal oxide resists are attracting attention as next-generation materials due to their high EUV absorption and etching resistance.

They have the potential to improve resolution and thin-film resistance through reaction mechanisms and small building blocks that differ from those of organic polymer types.

However, metal oxide-based materials also have issues such as:

• sensitivity

• defect density

• development process

• metal contamination control

• storage stability

• quality uniformity during mass production

.

While new materials have the potential to improve the RLS trade-off, they can also create other performance challenges.

In Part 8, we will cover these metal oxide resists in detail.

LER alone is not enough as an evaluation metric

Conventionally, material performance has been evaluated primarily based on three factors: resolution, LER, and sensitivity.

However, in EUV, that alone is not enough.

• LWR

• Local CD Uniformity (LCDU)

• Missing hole rate

• Bridge rate

• Post-etch defects

• Electrical yield

must also be evaluated.

imec points out the need to treat extreme stochastic failures as an independent metric in addition to the conventional RLS.

In other words, modern resist development has shifted from a "triangle" to multi-dimensional optimization that includes defects and pattern transfer performance.

The optimal solution varies depending on the application

The same resist is not necessarily optimal for every process.

Line and space

It is important to suppress LER, LWR, line breaks, and bridging.

Contact hole

Missing holes, hole diameter variation, and shape abnormalities become important.

High-density memory

Periodicity, uniformity, and a low defect rate are strongly required.

Logic gates and wiring

Local CD variation directly affects transistor performance and wiring resistance.

Therefore, material manufacturers do not maximize a single overall score, but rather change priorities for each customer process.

Some layers prioritize high sensitivity, while others prioritize LER or defects even if the exposure dose increases.

Challenges as of 2026

Even now, as the evaluation of High-NA EUV progresses, issues such as LWR and local CD uniformity in gate patterning remain direct factors affecting device variation.

Obtaining a high-resolution optical image is not enough.

The resist must be able to capture that image with low roughness and low defects, and then faithfully transfer it through to the post-etch stage.

Future competition will not simply be about developing high-sensitivity materials, but rather

.

Summary

Sensitivity, resolution, and LER are not independent performance metrics.

• Lowering the exposure dose increases photon noise

• Increasing acid diffusion improves sensitivity but degrades resolution

• Suppressing diffusion sharpens the profile but increases the required exposure dose

• Reducing molecular size can improve granularity, but it alters film and etching performance

As shown, these are linked through the same materials and reaction mechanisms.

However, this triangle is not an insurmountable barrier.

There is room to push performance limits outward through new PAGs, uniform material design, single-component systems, metal oxide resists, and co-optimization with underlayers.

is.

Next time, we will examine the potential for metal oxide resists, which are attracting attention as next-generation candidates, to replace organic resists.

References

• imec "Challenges and innovations in patterning"

• NIST "Advanced Metrology to Enable Next Generation EUV Photoresists"

• IBM Research "New Photoacid Generators Designed for Advanced EUV Patterning"

• IBM Research "IBM demonstrates High NA EUV process capability on logic features below the 2nm node"

• Review article "Line-Edge Roughness from Extreme Ultraviolet Lithography to High-NA EUV"

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#Semiconductor #Photoresist #EUV #LER #Resolution #Sensitivity #HighNA #SemiconductorMaterials