Development contrast

In recent years, demands on microlithographic techniques have become much more severe along with the reduced sizes of semiconductor devices. Various techniques to enhance the performance of microlithographic resist systems have been developed. Contrast enhanced lithography (CEL) is one... 

Resist contrasts are defined as the slopes of the linear portion of the sensitivity curves (Fig. 3) and depend on process conditions. Thus, the sensitivity (contrast) curves are constructed to semi-optimize process conditions for a given formulation. However, final optimization of a resist formulation and process conditions requires lithographic imaging of target features. A plot of a dissolution rate as a function of exposure dose (cf. Fig. 172) is very useful in assessing the developer selectivity (development contrast) as mentioned earlier. 

Another useful contrast values are related to the resist chemistry in the film, which will subsequently affect the lithographic contrast. Sensitivity (contrast) curves similar to Fig. 3 can be generated by following the degree of reaction (deprotection, for example) with IR or by measuring thinning (in deprotection, for example) as the function of exposure dose. Comparison of a chemical contrast curve with a development contrast curve provides useful information on resist behavior, such as a degree of deprotection at E0. 

Following exposure, poly(olefin sulfones) can be developed by two main methods by solvent development or by thermal development. The exposed areas of the resist simply evaporate on heating, or in some cases during exposure, in a phenomenon termed self-development, which negatively impacts the vacuum of the exposure tool s electron column. The liquid development method is not without its drawbacks, as it requires a careful choice of solvent, since the development contrast depends only on molecular weight. ... 

The addition of base quenchers to resists has been shown to improve LER at the expense of photospeed (see Fig. 17.28). This stems from the neutralization reaction between the photoacid and the base quencher within the exposed area, resulting in the increase in the photogenerated acid concentration gradient, as well as chemical contrast at the feature edge. The enhanced chemical contrast at the feature edge translates into enhanced development contrast between the exposed and the unexposed areas of the film the result is a much sharper edge (lower LER) than would otherwise be the case. 

Exceptions to Kasha s rule can not only be found with azulene and other compounds [37], where emission from S2 is observed (typically because the energetic difference between the S2 and Si states is sufficiently large to reduce the S2-S1 internal conversion to values close to the S2-S0 radiative rate), but also when there is competition between vibrational relaxation and photochemistry, the so-called vibronic effect. This is an important concept that has been recently developed contrasting with the general wisdom in photochemistry, that only very few exceptions to Kasha s rule exist. The foundations of the vibronic effects were found in 1966 when Ralph Becker and Joseph Michl noticed that the fluorescence excitation spectrum of a photochromic compound, 2,2-diethylchromene (see Scheme 15.4), was significantly different from the absorption spectrum [38]. 

In a retrospective case-cohort study in 809 patients who developed contrast-induced nephrotoxicity after either intraarterial or intravenous contrast administration and 2427 patients who did not,... 

The particular utility of NMR microscopy lies in the contrasts that are available. Image contrast in NMRI depends on material-specific parameters (spin-density and nuclear spin relaxation times), operator-related parameters (pulse sequence, pulse delay and repetition times) and external parameters (temperature, viscosity, etc.). Common contrast mechanisms in solid-state NMR imaging are based on relaxation times (T, T2, T p. T ) and chemical shifts. Most studies develop contrast based either on spin density or T2 differences since these show up immediately without the need of modifying the imaging sequence. The unsurpassed soft-matter contrast of NMRI is hard to achieve with competitive methods like X-ray or computer tomography. 

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