The acid generation mechanism of photoacid generators

1. Acid-generation mechanism of ionic photoacid generators (example of a sulfonium salt)

When a sulfonium salt-type photoacid generator is exposed to light, the cation absorbs the light energy, causing the C-S+ bond to break and a radical species to be generated. The generated sulfonium radical causes the formation of the conjugate acid of the corresponding counter anion by extracting protons from the system (reaction (i) in the figure below) or recombining with Ar radicals (reaction (ii) in the figure below).

2. Acid-generating mechanism of nonionic photoacid generators (example of imidosulfonate)

In imidosulfonate-type photoacid generators, the imide side structure is the light-absorbing site. The decomposition mechanism by light irradiation is basically the cleavage of the sulfonic acid ester, so the generated acid is limited to sulfonic acid.

ref) C.J.Martin,et al., J. PHOTOCHEM. PHOTOBIOL., C., 34 (2018) 41.

Examples of photoacid generator use

1.Guidelines for selecting an ionic photoacid generator

The acids generated from photoacid generators are used in various catalytic reactions. The table below shows the typical acid-catalyzed reactions classified by type.

Can be scrolled horizontally

Classification	Type of reaction	Summary
Polymerization	Ring-opening polymerization	Ring-opening polymerization of oxirane rings such as epoxies and oxetanes
	addition polymerization	Cationic polymerization of unsaturated bonds such as vinylethers
Cross-linking	Condensation reaction	Condensation reaction between phenolic resins and cross-linking agents
		Condensation reaction of silanol compounds
Deprotection	Deprotection reaction	Deprotection reaction of protective groups such as phenolic hydroxyl groups and carboxylic acid groups

ref) Takumi Ueno, New Developments in the Development of Photoresist Materials (CMC Publishing), Chapter 5

Among these acid-catalyzed reactions, the following three have a wide range of industrial applications: (1) cationic polymerization of epoxy resins, (2) deprotection of polyhydroxy styrene resins (resins containing phenolic hydroxyl groups), and (3) cross-linking reactions using phenolic resins and trimethylol melamine as cross-linking agents.

(1) Cationic polymerization of epoxy resin

In the case of cationic polymerization of epoxy resin, the acid generated from the photoacid generator acts as the acid that initiates cationic polymerization.
The reaction mechanism of cationic polymerization of epoxy resin is shown below.

First, a photoacid generator releases acid when exposed to light. The acid that is generated coordinates with the epoxy to produce an active species for cationic polymerization.
Then, the active species undergoes ring-opening by nucleophilic attack of another epoxy, and the second active species, oxonium cation, is generated.After that, sequential ring-opening polymerization of epoxy and oxonium cations progresses, resulting in epoxy resin.

(2) Deprotection reaction of polyhydroxystyrene resin (resin containing phenolic hydroxyl groups)

Acid-catalyzed deprotection reactions are mainly used in the field of photolithography technology, and are put to practical use in chemical amplification resist resins used in the manufacture of semiconductor devices.
One specific example is a resist resin in which a photochemical acid generator is co-existing with a polyhydroxystyrene resin to which a protective group such as t-Boc has been introduced.

In general, polyhydroxystyrene resin is soluble in alkaline aqueous solutions when the phenolic hydroxyl groups (OH groups) are unprotected.
On the other hand, when protective groups such as phenolic hydroxyl groups t-Boc are introduced, the resin becomes insoluble in alkaline aqueous solutions.

In resist resins that contain a photoacid generator in combination with such a resin system, the photoacid generator causes acid generation only in the area exposed to light, and the resulting acid causes a deprotection reaction. This makes it possible to pattern the resist using an alkaline development process, as only the resist resin in the area exposed to light becomes soluble in an alkaline solution. This technology is applied to photolithography in the form of chemically amplified photoresist for semiconductor manufacturing.

The reaction mechanism of the deprotection of the t-Boc protective group is shown below.

The acid generated by the photoacid generator undergoes protonation at the oxygen of the t-Boc group's carbonate. The protective group is then removed through decarboxylation, and the phenolic hydroxyl group is regenerated at the same time as the acid is regenerated.

(3) Condensation-type cross-linking reaction using phenol resin and trimethylol melamine as cross-linking agents

The most well-known condensation-type cross-linking reaction using acid catalysts is a three-component system composed of phenol resin, trimethylol melamine compound (cross-linking agent) and photoacid generator.
The reaction mechanism is shown below.

The acid generated from the photoacid generator adds to the oxygen atom of the methol group in the cross-linking agent and is released as methanol. The carbocation produced by this reaction undergoes an electrophilic addition reaction with aromatic rings (phenolic resin) that have electron-donating substituents.

Using multifunctional methol compounds increases the number of cross-linking points, improving the physical properties of the cured material.

Guidelines for selecting photoacid generators

1. Guidelines for selecting an ionic photoacid generator

Guidelines for selecting a cationic group
When selecting an ionic photoacid generator, the cationic group should be considered first.
The cationic group of an ionic photoacid generator determines which wavelengths of light are absorbed to what extent, so it is necessary to select a c
The unit used to express the degree of light absorption is the molar extinction coefficient ε (unit: mol-1 L cm-1).

In general, if the light absorption of the cationic site at the light source wavelength is completely zero, no acid generation will occur at all. As the light absorption increases (i.e. as the However, in the case of thick films in particular, if the molar absorption coefficient is too high, the light will be absorbed only at the surface of the film and will not reach the deeper layers, which can cause curing defects.

For example, as shown in the diagram below, when using a photoacid generator with ε=10,000, only about 10% or less of the light reaches a point 50μm deep from the surface of the film, so curing defects are likely to occur in the deep layers of the film at a thickness of 50μm, etc.

On the other hand, at a film thickness of around 10μm, the effect of the size of the molar absorption coefficient on the transmittance is small, so using a photoacid generator with a high ε value will increase the amount of light absorbed and increase the reactivity.

Conditions
・Molecular weight of PAG : 1,000
・Amount of PAG : 2.5wt% (per resin)

Conditions
・Molecular weight of PAG : 1,000
・Amount of PAG : 5.0wt% (per resin)

The optimal cationic site and degree of light absorption will vary depending on the type of material you want to apply it to and the thickness of the film, so please contact us for advice.

UV-vis absorption spectra of main ionic photoacid generators

The figures show the results of measuring the absorption spectra of the CPI series and IK-1 series in acetonitrile solution.
・The CPI-100 and 200 series can be used as general-purpose grades.
・The CPI-300 series can be used as i-line high-sensitivity types.
・The CPI-400 series can be used as a high-sensitivity gh-ray type.
・The IK-1 series can be used as a general-purpose grade when used independently, and can also be used for longer wavelengths when combined with a sensitizer.

Guidelines for selecting anions

Once you have selected the cation, you will need to select an anion that is suitable for the reaction system.
The following are some points to consider when selecting an anion. The most suitable anion will differ depending on the type of material you want to use and the reaction type, so
please contact us for advice.
・Acid strength
・Solubility in resins and solvents
・Diffusivity
・Color change after light irradiation (over time)
・Compliance with the Poisonous and Deleterious Substances Control Law (in particular, whether or not antimony is contained)

The graph shows the results of an experiment comparing the epoxy curing reaction of the CPI-100 and 200 series with different anions.
As the epoxy curing reaction progresses, the pencil hardness of the cured resin increases, so it is possible to evaluate the amount of photoacid generator added and the reactivity of the epoxy curing reaction.
・For the PF6 anion, a large amount of PAG needs to be added in order to promote the epoxy curing reaction.
・For the SbF6 anion and PF3(C2F5)3 anion, a small amount of PAG can be added to promote the epoxy curing reaction.
The difference in reactivity is determined by the acid strength of the generated acid.
If the acid is weak, the acid is trapped by the ether or ester structure in the resin and the reaction stops, so the higher the acid strength, the more reactive it is.

Comparison of curing properties using anions with epoxy curing reactions
Experimental conditions
・Cationic component: CPI-100, 200 series
・Light source: Metal halide (120 W/cm2

The above diagram is a summary of the anion selection points.
・The further to the right in the diagram, the more strongly acidic the anion.
・The further up in the diagram, the more highly soluble the photoacid generator that can be made by combining anions.
・The volume (calculated value) of the anion is shown below each anion, and the smaller the volume, the more diffusible the acid generated.

Function of photoacid generator (Youtube video)

Video Overview
Function of Photo Acid Generator (PAG)
Rhodamine coloration experiment 1
A: Solution without PAG B: Solution with PAG Exposure comparison experiment
Rhodamine coloration experiment 2
A:Commercial PAG　B:Exposure comparison experiment of i-line high-sensitivity PAG