Photoinitiators and Photosensitizers

All of the negative-working ED photoresists contain a photoinitiator and, in some cases, a photosensitizer as well. The photoinitiator is a compound that absorbs incident UV radiation of an appropriate wavelength from the exposure lamp during the imaging step and splits into free radicals, which initiate polymerization of the unsaturated monomer (or unsaturated polymer) (see e.g. Fig. 2.12). 

Sometimes light of the appropriate wavelength for initiating the above reaction with a particular photoinitiator is unavailable in sufficient intensity from the standard exposure units used. In these cases another photo-active compound is added with the photoinitiator this is usually referred to as a photosensitizer. The photosensitizer is chosen so that it absorbs the appropriate wavelength of light and also is matched to the photoinitiator in a way that it can transfer the absorbed energy to the latter, causing it to split into active radicals. Section 2.4.4(b) describes the mechanism in detail. 

The choice of photosystem varies greatly from one resist to another however, it must be compatible with the other resist components and not separate from them on standing or during electrodeposition. Sometimes a compound that acts as a photoinitiator in one resist can function as a photosensitizer in another. Examples of the more common photoinitiators are -hydroxyisobutylphenone, benzoin ethyl ether, 2-t-butyl- 

Virtually all of the photosystems used in ED photoresists are sensitive to UV light. However, a composition has been reported that is sensitive to the visible radiation of an argon ion laser at 488 nm [19]. This employs the compound shown in Fig. 2.13 as a photosensitizer. 

The photoinitiator is chosen so that its peak absorption wavelengths are matched with the major wavelengths emitted from the mercury vapour lamp to give maximum photocrosslinking. If the photoinitiator wavelength is insufficiently close to a major mercury emission wavelength, there are two ways to improve efficiency. First, a doped mercury vapour lamp can be used that has a more appropriate spectral output relative to the absorption of the photoinitiator. Metal halides are usually used to accomplish this. However, medium-pressure mercury lamps 

There are essentially two types of compounds that are used in the UV curing process to absorb the light and generate reactive species. These are photoinitiators and photosensitizers. 

photosensitizers are useful mainly by being capable of extending the spectral sensitivity of certain photoinitiators under specific conditions. 

In the UV curing process, photons from the UV source are absorbed by a chromophoric site of a molecule in a single event. The chromophore is a part of the photoinitiator. The light absorption by the photoinitiator requires that an emission light from the light source overlap with an absorption band of the photoinitiator. 

The photon absorption follows Lambert-Beer s law (see above). The number of photons I present at depth 1 from the surface is given as a function of the optical absorbance, A, normalized to the initial number of photons, Iq  

In general, upon exposure to UV radiant energy, a photoinitiator can generate free radicals or ions, as pointed out earlier. These are generated at a rapid rate, and their depth profile corresponds to the inverse photon penetration profile. Similar to electron penetration, the final cure profile often deviates from the initial radical or ion distribution, since they can live much longer than the exposure time. The mechanisms of the processes for the generation of reactive species are discussed in detail in Davidson.  

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Materials. For holographic information storage, materials are required which alter their index of refraction locally by spotwise illumination with light. Suitable are photorefractive inorganic crystals, eg, LiNbO, BaTiO, LiTaO, and Bq2 i02Q. Also suitable are photorefractive ferroelectric polymers like poly(vinyhdene fluoride-i o-trifluorethylene) (PVDF/TFE). Preferably transparent polymers are used which contain approximately 10% of monomeric material (so-called photopolymers, photothermoplasts). These polymers additionally contain different initiators, photoinitiators, and photosensitizers. 

Approximately half of the papers in the book deal with photopolymerization reactions including the role of photoinitiators and photosensitizers. The other half of the papers deal with photodegradation reactions including the use of photostabilizers and photodegradable polymers. 

The finding of a best method for introducing photoinitiators and photosensitizers into polymers is a very important practical problem. Two main methods are in this case applied a polymer film is cast from a solution with the respective photosensitizer added,or the photosensitizer is pressed into the film at an elevated temperature. In the first method it is sometimes very difficult to remove all traces of solvent,which may influence the photoreactions observed. In the second method conditions of pressing (temperature of 100-200°C and pressure of 100-200 atm) may alter the polymer and compound added. 

However, the two terms photoinitiator and photosensitizer are often used in a more general sense to define any process involving either energy transfer (to generate from I the same radicals as those obtained through direct excitation) or electron transfer followed by proton transfer (to form new initiating radicals) Scheme 1). 

Therefore, by analogy with the term initiator in thermal polymerizations, the dye is also usually presented as a photoinitiator (PI), i.e. a substance that absorbs light and participates in the photoinitiation of a polymerization reaction. In die sense of photochemistry [TUR 90], however, it can also play the true role of a photosensitizer (PS) in some specific reactions. Sometimes, there is an apparent ambiguity with the words photoinitiator and photosensitizer in papers dealing with... 

Superfast curing (in less than 0.5 s) of acrylate adhesives for plastics has been achieved by a proper choice of prepolymers, photoinitiators, and photosensitizers and successfully used in blister packaging, " as well as for bonding glass to plastics ... 

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