Reinhold Schwalm

Overcoming oxygen inhibition in UV-curing of acrylate coatings by carbon dioxide inerting: Part II

The inhibitory effect of molecular oxygen in the photoinitiated polymerization of acrylate resins has been completely eliminated by operating in a carbon dioxide atmosphere. The high speed polymerization was followed in situ by real time infrared spectroscopy, thus allowing conversion versus time curves to be recorded for curing reactions occurring within a fraction of a second. The influence of the O2 concentration and of the sample temperature on the polymerization kinetics has been quantified for a polyurethane-acrylate resin. Replacing air by CO2 proved to be particularly beneficial for polymerizations carried out under conditions where O2 diffusion was enhanced, i.e. thin films of a highly fluid resin exposed to low intensity light at high temperatures.

Tuning the mechanical properties of UV coatings towards hard and flexible systems

The standard resins of radiation-curable coatings provide either hard or flexible coatings dependent on the type of chemistry used. Whereas aromatic epoxide acrylates usually give hard and brittle coatings, urethane acrylates are known for their flexibility. Since the radiation curable systems should not contain solvents, the desired low viscosity for the specific application is adjusted with reactive monomers. This normally prevents the use of flexible high-molecular-weight polymers. On the other hand, the viscosity of dispersions is determined by the solid content only and not by the molecular weight of the polymers used. Thus, waterbased UV-curable coatings are one strategy out of this dilemma in order to combine the flexibility of higher-molecular-weight polymers with the hardness of highly crosslinked acrylates. The mechanical data of conventional and waterbased UV coatings are discussed in dependency on glass transition temperature and elastically effective chain length between crosslinks.

UV-radiation curing of waterbased urethane-acrylate coatings

A kinetic study of the drying and UV-curing of waterbased urethane–acrylate formulation was conducted by means of infrared spectroscopy. The water release upon heating of the dispersion was shown to depend on the temperature and on the film thickness. The drying of a 30 μm thick coating was completed in <2 min upon heating in an oven at 80°C. The influence of a number of critical factors on the polymerization kinetics has been investigated, namely the type of photoinitiator, the sample temperature, the chemical structure of the functionalized oligomer and its acid content. As expected, the latter factor has a strong effect on the properties of the UV-cured coatings, in particular their hardness and their hydrophilic character. The water uptake and the softening of coatings placed in a wet atmosphere was found to be directly related to the carboxylic acid content of the urethane–acrylate polymer. This phenomenon is fully reversible and does not affect the long term properties of the UV-cured coatings.

Depth profiling of UV cured coatings containing photostabilizers by confocal Raman microscopy

The chemical imaging possibility of confocal Raman microscopy was used to characterize UV cured coatings layers. Depth profiles of acrylate curing conversion were recorded in order to elucidate the interaction of photoinitator, photostabilizer, and irradiation source. Formulations containing acyl phosphine oxides resulted in a better through cure.

Depth-resolved characterization of UV cured coatings by confocal Raman and two-photon microscopy

UV cured coatings can be characterized very effectively by confocal Raman microscopy, real-time IR spectroscopy and two-photon laser scan microscopy. The first two vibrational spectroscopic methods for chemical imaging and kinetics of curing conversion, respectively, can help to optimize UV formulations and curing conditions, e.g. UV dosage or temperature. In contrast, two-photon laser scan confocal microscopy can be used to monitor changes in refractive indices and intrinsic or tracer fluorescence with three-dimensional spatial and fast temporal resolution. This technique is well suited to analyze buried interfaces to substrate and other coating layers and to visualize distribution of fillers. In this regard, aluminum flakes in metallic coatings can be investigated without additional sample preparation as a function of their individual orientation and size. With this information, one can optimize the process of coating formation yielding improved coloristic properties. Illustrative examples will be presented to elucidate the perspective of these techniques in coatings characterization.

A new environmentally stable protective group for deep UV resists: Methoxy(tetrahydropyranyl) ether

Deep UV photoresists are designed to be used in the manufacturing of highly integrated chips (>16 Mbit). They differ from the conventional photoresists in their principal chemistry. The vast majority of positive deep UV resists are based on protected poly(hydroxystyrene) resins and photochemical acid generators (PAG). They rely on photochemically induced acid-catalyzed reactions (chemical amplification) to generate the desired pattern and meet the high-sensitivity requirements. It turned out that the type of the acid labile protective group is of paramount importance for the performance of the resists. It has to be stable enough not to be cleaved by the weakly acidic phenol at room temperature, but has to be labile enough to be cleaved readily even at the top of the resist where portions of the generated acid may be neutralized by airborne bases. Selection criteria for useful groups and the performance of the very well suited protective group methoxy(tetrahydropyran) are described in this paper.

Deep-UV resists with improved delay capabilities

In this paper, we present our approaches to solve the delay problem, i.e., the formation of T- tops during the delay time between exposure and post-exposure bake. We investigated the distribution of the photoacid generator in the resist layer and the influence of basic contaminations in the formulation. T-topped patterns can be avoided by using appropriate developers. The incorporation of additives make the resist less sensitive to basic contaminations. Some formulations show no T-top formation under severe conditions for a considerable amount of time.

Robust and environmentally stable deep UV positive resist: optimization of SUCCESS ST2

By optimization of SUCCESS ST2 an environmentally stable and robust deep UV positive resist has been derived where the generally encountered problems related to chemical amplification resists, the formation of T-tops and linewidth changes during delay have been solved. The previously reported chemistry, protected poly-p-hydroxystyrene and SUCCESS type sulfonium salts, has been proven to be insensitive to airborne contaminations. In this paper the optimization of processing conditions, based on thermal analysis is reported. With the derived conditions linewidth changes during delays could be minimized and excellent performance obtained. At the IBM lithography test center in Boblingen an integrated photosector, consisting of equipment, materials, process and control philosophy, was balanced and 0.25 micrometers pattern can routinely be resolved using an ASM-L DUV stepper (NA 0.5).

Dissolution inhibition-type photoresists for deep-UV lithography

A short introduction to photolithography with an emphasis on the classification of photoresists and the basic chemistry of conventional resists is given. The chemistry of positive type photoresists used for deep-UV lithography is reviewed and new developments in this field presented. The introduction of novel sulphonium compounds (SUCCESS) to inhibit the dissolution of poly(p-hydroxystyrene), the use of non-photoactive dissolution inhibitors together with acid generators and the protection of hydroxy groups in poly(p-hydroxystyrene) to decrease its solubility present ways of obtaining resists with high sensitivity and resolution.

MID UV Resist Materials Containing Pyridinium Ylides

Polymers containing pyridinium-ylide units were prepared as water soluble MID UV resist materials. The absorption maxima of the N-iminopyridinium ylides are centered around 320 nm. Upon exposure the ylide sensitizer bleaches cleanly and undergoes two competing reactions, leading via a singlet state to 1,2-diazepines and via a triplet state to pyridine and nitrenes. Both reactions are accompanied by a change in polarity and can be used in the application as photoresists. This pronounced polarity change renders the polymers hydrophobic and water repellant. Therefore negative images can be obtained by development with pure water. Due to a quantum yield below 0.1 the sensitivity is low, but can be enhanced by addition of sensitizers.