Photoresists modeling

A.R. Neureuther and F.H. Dill, Photoresist modeling and device fabrication applications, in Optical and Acoustical Micro Electronics, pp. 233 249, Polytechnic Press, New York (1974). 

Photopolymerizable coatings relief-image-forming systems, 6,125 Photoreactivity environmental effects, 1, 394 Photoredox properties bipyridyl metal complexes, 2, 90 Photoresist systems, 6,125 Photosensitive materials, 6, 113 Photosynthesis anoxygenic, 6, 589 magnesium and manganese, 6, 588 water decomposition models, 6, 498... 

Novolac resins, as the oldest synthetic polymers, have played an important role 1n microelectronic Industry as positive photoresists. Studies of novolac dissolution have populated the literature a recent survey shows that the rate of dissolution 1s influenced by the concentration of the alkali, size of the cation, addition of salt, and the presence of dissolution Inhibitors (1-6). The voluminous experimental results, however, have not led to a clear understanding of the dissolution phenomena. Arcus (3) proposed an 1on-permeab1e membrane" model while Szmanda (1) and Hanabata (6) emphasized the Importance of secondary structures of novolac molecules, for Instance, Inter- or Intramolecular hydrogen bonding and the various isomeric configurations of the resins. These important contributions nevertheless point to a need for additional studies of the mechanism of dissolution. 

CARS microscopy has emerged as a highly sensitive analytical tool for vibrational bioimaging, predominantly, of lipids in membrane model systems [69, 81-84], live unstained cells [85-95, 43], and both ex vivo and in vivo tissues [26, 96-103, 43]. Examples of CARS imaging applications in the physical and material sciences include the study of fracture dynamics in drying silica nanoparticle suspensions [104], patterned polymeric photoresist film [105], drug molecules in a polymer matrix [106], and liquid crystals [107, 108],... 

On one hand it is investigated how the addition of cationic surfactants affects the pattern collapse of 193 nm photoresist lines. On the other hand, the adsorption of the surfactant on model photoresist surfaces is explored by a variety of surface chemical methods. Of special interest is how the surfactant changes the surface properties of the photoresist as surface potential and wettability. For an optimum modelling of the properties of real photoresist structures, both unexposed photoresists and photoresists that have been UV exposed, baked and developed are studied. 

Physico-chemical characterization. For the physicochemical characterization of the photoresist, flat model surfaces are needed that are accessible for the methods described below. They were prepared by spin-coating the photoresist directly onto a silicon wafer with a native oxide layer. Then, they were exposed to DUV radiation with various doses by open frame exposure. The exposed wafers... 

Dry photoresist layers. The capillary forces causing pattern collapse act on the vertical sidewall of the photoresist structures. These surfaces are formed during the development at the interface between soluble and insoluble photoresist. It is generally accepted that a photoresist with a deprotection level above 80% is soluble in the developer [3], Therefore it can be concluded that the structure sidewalls consist of a blend of more than 20% protected and less than 80% deprotected photoresist. Because of their small dimensions, the sidewalls are not accessible for most surface chemical methods. To allow an investigation, they have to be modelled by flat photoresist surfaces. To simulate the properties of the real sidewalls, these surfaces have to be processed, i.e. exposed, baked and developed. One goal of this study is to find out which of the flat... 

Furthermore we concluded that the photoresist layers processed with 100-110% of the threshold dose reflect quite well the properties of the structure sidewalls in the photolithographic process. The surfactant treatment of these model photoresist layers should be comparable with the surfactant rinse in the photolithographic process. Therefore the hydrophobizing of these layers can serve as a proof of the hydrophobizing of the photoresist structures at surfactant concentrations around ceff- In our experiment,... 

Nevertheless a good coincidence between the reduction of the capillary forces under laboratory conditions with the reduction of pattern collapse in the photolithographic process was found. This supports our hypothesis that cationic surfactants are able to reduce pattern collapse by hydrophobizing the photoresist surface. It shows also that the hydrophobizing must occur in a similar way under real conditions. Thus, flat photoresist layers processed at the threshold dose are appropriate model surfaces for the sidewalls of photoresist structures. 

Goal of the present study was to investigate how solutions of cationic surfactants affect the pattern collapse in the photolithographic sub-100 nm structuring. On one hand, the solutions were applied directly in the photolithographic process to investigate their ability to reduce the pattern collapse. On the other hand, the adsorption of the surfactant on flat model photoresist layers was studied using a variety of physicochemical characterization methods and its influence on the capillary forces was determined. 

The physico-chemical characterization of processed flat photoresist layers showed that the properties of the photoresist change significantly if it is processed with the threshold exposure dose that makes the photoresist soluble in the developer. Caused by a deprotection reaction, the photoresist becomes acidic and more hydrophilic. It was postulated that the thin residual photoresist layers processed with the threshold dose are appropriate model surfaces for the sidewalls of the photoresist structures in the photolithographic process. 

The minimum of the capillary forces in the model system correlates with the maximum of the pattern collapse reduction in the real photolithographic process. It is concluded that the sidewalls of the photoresist structures are hydrophobized in a similar way as the model surfaces by the cationic surfactant rinse even although the conditions in the process differ from those of the characterization experiments. The resulting reduction of the capillary forces during the drying of the structures is the essential factor in the reduction of pattern collapse. 

Reactive Ion Etching. Etching experiments were carried out in an Applied Materials Model 8110 Hex reactor. Alternatively, a Cook Vacuum Products Inc. Model C71 RF/DC Sputtering Module was employed. Film thickness measurements were taken before and after etching to determine etching rates, and the rates were typically compared to that of the novolac-diazoquinone photoresist, HPR-206, baked at 210 C for 1 hour. Measurements were obtained on a Dektak Model IIA profilometer. 

For positive photoresists the model of Dill ( ) is used to simulate exposure and development. In this model the illumination intensity I at 248 nm and the normalized concentration of photoactive compound are given by the two coupled equations,... 

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