[Photoresist Series Part 4] Why Chemically Amplified Resists Changed the Semiconductor Industry
Looking back at the history of photoresists, one of the biggest turning points that enabled miniaturization from KrF onwards ischemically amplified resists.
Before the advent of this technology, lithography materials were bound to a world where "the reaction occurs only in proportion to the amount of light exposure."
However, chemically amplified resists changed that conventional wisdom.
Triggering multiple chemical reactions from a single photoreaction
This concept of "amplification" made it possible to obtain significant solubility changes even with low exposure doses, paving the way for miniaturization that continues through KrF, ArF, and EUV.
In this installment, we will delve into why chemically amplified resists changed the semiconductor industry, from their reaction mechanisms to their significance in mass production.
What is a Chemically Amplified Resist?
A chemically amplified resist is a resist that uses a small amount of chemical species generated by exposure as a starting point to trigger a chain of numerous molecular reactions during the subsequent baking process.
For typical advanced positive-tone resists, the following composition is fundamental:
Acid-protected polymer
Photoacid generator (PAG)
Solvent
Quencher
Additives for interface control and defect suppression
A key role in exposure is played by the Photoacid Generator (PAG).
When the PAG receives light, it generates acid. This acid acts as a catalyst during the subsequent post-exposure bake, promoting the deprotection reaction that removes the polymer's protecting groups.
As a result, the exposed areas become more soluble in alkaline developer, allowing for the formation of fine patterns.
Why is "amplification" important?
In conventional non-chemically amplified resists, the chemical change triggered by a single photon was relatively limited.
However, as the exposure wavelength shortened to 248 nm for KrF and 193 nm for ArF, higher sensitivity became necessary.
The reason is simple: exposure time cannot be extended on mass production lines.
Exposure equipment is extremely expensive, and if the time required to process a single wafer increases, the productivity of the entire factory drops.
What was needed then was a material that reacts sufficiently even with a low exposure dose.
In chemically amplified resists, the acid generated by a single exposure acts as a catalyst and repeatedly participates in the reaction.
In other words,
1. Light generates acid
2. That acid removes the protecting group
3. The solubility of the exposed area changes significantly
This sequence proceeds very efficiently.
This structure is the essence of chemically amplified resists.
Reaction mechanism of chemically amplified resists
A simplified explanation of a typical positive-type chemically amplified resist is as follows.
1. Before Exposure
The polymer has protective groups attached, making it resistant to alkaline developers.
2. Exposure
The PAG absorbs light and generates acid.
3. Post-Exposure Bake (PEB)
The generated acid removes the protective groups from the polymer. This is called a deprotection reaction.
4. Development
The deprotected exposed areas become more polar, making them easier to dissolve in alkaline developers.
5. Pattern Formation
The unexposed areas remain, and only the exposed areas are removed.
What is important here is that light does not directly create the pattern.
In reality, exposure only provides the trigger for the reaction, and the final change in solubility is significantly amplified by the chemical reaction during PEB.
Why is the PAG so important?
The PAG is the heart of the chemically amplified resist.
An ideal PAG is required to have the following performance:
Capable of generating sufficient acid with a low exposure dose
The generated acid is stable
Does not migrate excessively within the resist film
Can suppress unnecessary side reactions and outgassing
Difficult to decompose during long-term storage
In other words, the PAG is not just a component that "needs to release acid."
It is a material that directly affects the final pattern quality, including the strength, distribution, and mobility of the generated acid.
Especially at advanced nodes, the choice of PAG affects sensitivity, roughness, defects, line edge roughness, and CD variation.
PEB is not just a heating process
Post-Exposure Bake (PEB) is often misunderstood as merely a "heating process".
However, in reality, PEB is the main battlefield for reactions in chemically amplified resists.
The main things that happen during PEB are as follows:
Acid diffusion
Deprotection of protecting groups
Change in polymer polarity
Expansion of the reaction region
Formation of reaction termination conditions
If the PEB temperature is too low, the reaction will be insufficient, and sensitivity will decrease.
Conversely, if it is too high, the acid will diffuse too widely, causing the pattern boundaries to blur and the dimensions to become unstable.
In other words, PEB is not only a "process for advancing the reaction" but also a "process for preventing the reaction from running out of control."
The greatest advantage of chemically amplified resists
The biggest reason why chemically amplified resists changed the semiconductor industry is that they greatly increased the possibility of achieving both sensitivity and resolution.
The main advantages are as follows:
Can create large solubility changes with low exposure doses
Contributes to improving the throughput of expensive exposure equipment
Compatible with advanced light sources such as KrF, ArF, and EUV
Allows for diverse optimization through molecular design
Enables fine-tuning of reaction designs necessary for miniaturization
This established lithography at a level where it is not just "possible to draw," but "possible to mass-produce."
What is required in a semiconductor factory is not laboratory-level success, but the stable processing of millions of wafers.
Chemically amplified resists were a technology that met those practical demands.
Yet it is not simple—the fate of acid diffusion
While chemically amplified resists are an excellent technology, they are not a panacea.
Rather, their essence of "amplification" creates new difficulties.
The most famous challenge is acid diffusion.
If the acid moves too widely, the boundary between exposed and unexposed areas becomes blurred.
This leads to:
line widths becoming thicker than planned
adjacent patterns connecting
contact holes deforming
increased line edge roughness
local defects increase
and other such problems occur.
The more you want to increase sensitivity, the more you want to strengthen the reaction.
However, if the reaction is made too strong, dimensional control becomes difficult.
This relationship is the so-called sensitivity-resolution-roughness trade-off.
Why is a quencher necessary?
In chemically amplified resists, simply generating acid is not enough.
You must also control how far the generated acid moves and where it stops.
A quencher is used for that purpose.
The quencher acts as a basic component, neutralizing excess acid and suppressing the spread of the reaction area.
Their roles can be summarized as follows:
Suppressing excessive acid diffusion
Reducing blurring at pattern edges
Suppressing line edge roughness
Reducing minute chemical noise
However, increasing the quencher too much will decrease sensitivity.
In other words, PAGs and quenchers are not opposing components, but rather two wheels working together to perfectly balance the reaction.
The role played by chemically amplified resists in ArF
Chemically amplified resists, which demonstrated their practicality in the KrF era, became even more important in the ArF era.
At 193nm, the polymers used for 248nm cannot be used as they are.
A new polymer design that balances optical properties, transparency, alkali developability, and etching resistance became necessary.
Therefore, chemically amplified resists evolved as an integrated technology of multiple elements, such as
polymers with high 193nm transparency
appropriate PAGs
quenchers that control acid diffusion
surface designs that can withstand immersion processes
and so on.
The achievement of higher resolution in the ArF immersion era was made possible not only by equipment, but also by the refinement of chemically amplified resists.
Are chemically amplified resists reaching their limits in the EUV era?
Even in the EUV era, chemically amplified resists remain the mainstream.
However, the situation becomes even more complex with EUV.
13.5nm light is high-energy, and secondary electrons are involved after light absorption. As a result, the reaction becomes more stochastic, and minute variations are more likely to manifest as significant defects.
Typical issues with EUV are as follows.
Stochastic defects
Micro-bridging
Missing holes
Line edge roughness
Reaction variations due to low photon counts
Chemically amplified resists are also effective here, but at the same time, new material theories are emerging, such as 'how much acid diffusion should be allowed' and 'whether to move toward single-component or metal-containing systems.'
In other words, chemically amplified resists are not a finished product, but a technology that is still evolving.
Why this technology changed the semiconductor industry
The reason chemically amplified resists changed the semiconductor industry is not just about improved sensitivity.
This technology became a 'bridge' for converting the capabilities of exposure equipment into mass production processes.became a 'bridge' for converting the capabilities of exposure equipment into mass production processes..
If it were not for chemically amplified resists,
the practicality of deep ultraviolet lithography would have been low
exposure times would have been too long
factory throughput would not have been sufficient
achieving both miniaturization and mass production would have been difficult
is a possibility.
In other words, while this material technology tends to be overshadowed by exposure equipment, it is actually a central player in semiconductor miniaturization.
While EUV exposure equipment is in the spotlight, it is still the resist material that makes it possible to actually draw circuits using that light.
Summary
Chemically amplified resists,
generate acid with PAG
and advance the reaction with PEB
Changing solubility through deprotection
Creating fine patterns through development
This mechanism triggers a large chemical change from a small amount of exposure.
This concept of "amplification" took semiconductor miniaturization to the next level from the KrF era onward.
On the other hand, it also carries challenges such as acid diffusion, roughness, defects, and stochastic effects.
Even so, the reason this technology has remained the mainstream for decades is that
it offers an extremely high balance of miniaturization, mass production, and material design flexibility
.
Chemically amplified resist can be called a prime example of a "material invention" that changed the history of the semiconductor industry.
Next time, I will explain ArF immersion resist, born from the refinement of chemically amplified resist, and the world of double patterning that pushed the limits of resolution.
#Semiconductor #Photoresist #ChemicallyAmplifiedResist #PAG #EUV #ArF #SemiconductorMaterials #Lithography