Calixarene resists

In recent progress, calixarene resists have been prominent [105]. Hexaacetate p-methylcalix[6]arene was demonstrated to work as a high-resolution negative resist. This resist also shows high etch resistance. Calixarene resist has an advantage in its molecular size (about 1 nm). Liquid crystal resists and inorganic resists show high resolution [106]. These resists are suitable for the fabrication of nanostructures. However, the resist sensitivities are lower than those of chemically amplified resists, even PMMA. 

We have studied EB drawing to form very fine dot arrays with fine pitch for 20-30 nm pitched dot arrays pattern using EB drawing with calixarene resist [2] on silicon wafer as the ideal substrate [3-7]. Furthermore, we have to form the fine dot arrays with fine pitch on practical material and substrate because there is a substrate dependence on the formed dot size and dot array pitch because various element dependence on primary electron scattering and charge-up occur as known well. 

We performed EB drawing of the fine pitch dot arrays on Si substrate using calixarene resist. Highly packed dot arrays patterns with a pitch of 30x30,25x25 and 20x20 nm have been fabricated. 

Some fabrications of "mesoscopic devices using calixarene resists were also carried out. We have reported a nano-dot array fabrication on Si substrate having dots 15 nm in diameter and 35 nm pitch, and discussed its resolution limit (7). 

Calixarenes are prospective candidates of materials for microelectronics. As electron resists, their small and round shape gives ultra-high resolution. Synthesis is comparatively easy starting from very inexpensive substances. Films of calixarenes were made with excellent quality, high heat resistivities and flat surfaces. Pattern fabrications are carried out easily with conventional processes. Thus, calixarene resists provide a practical means to fabricate nanos-tructures down to around 10 nm. It would be a great help to resarch and development of " mesoscopic devices . 

To support the NMR evidence, the number of arms in a PIB star was determined by a chemical method. Accordingly, the product of a representative experiment (entry 2 Table 1) was purified (fractionation) and subjected to core destruction. This technique has been repeatedly used in our laboratories to determine the number of arms of stars with aromatic cores and PIB arms [38,61,62,66]. In core destruction the aromatic cores are selectively destroyed by exhaustive oxidation while the saturated aliphatic arms resist oxidation and can be quantitatively determined by GPC (see Scheme 4). Experimentally, it was found that the core was completely destroyed after 16 h under the conditions used (single peak by GPC analysis, see Fig. 4). Control experiments, conducted under similar conditions, showed (by GPC) that calixarenes are destroyed by oxidation [61] and form low molecular weight products, whereas linear PIB survives the oxidation [66]. 

We used high resolution resists of ZEP520 (Nippon Zenon Co.) and calixarene (Tokuyama Co.) as positive and negative resists, respectively. Furthermore, we used very thimer resists. Their thicknesses are about 70 nm and 15 nm, respectively. The thicknesses were determined by whether we observe... 

Figure 2. SEM image of ultrahigh packed dot arrays resist pattern (a) and resist dot size variations (b) using calixarene (28-44 mC/cm 30 kV, a pitch of 25 nm x 25 nm).

Calixaren negative resist is suitable for formation of fine dot arrays compared with ZEP520 positive resist. 

The hexaacetate of 6 has been tested as a high resolution negative resist for electron beam lithography, showing sufficient resolution to be useful for nanoscale device processing. Quantum confined cadmium sulfide clusters are stabilized by the Mannich base calixarenes 195 (R = Me, Bu, cyclohexyl). ... 

Sensitivity for calixarenes are comparatively low compared to today s standard electron resists. This is natural because small molecules require many cross-links per unit volume to make a gel state (17). To improve the sensitivity, a chloromethyl group was selected because its reaction is a step-wise reaction. Introducing a reactive group which undergoes a chain reaction, such as an epoxy group, or employing a di sion mechanism in resist reaction was deliberately avoided to maintain the high resolution capability of calixarenes (18,19). 

To realize finer fabrications having the dimensions down to few nm, calixarenes are no more appropriate resists as they have around Inm diameter. It will be required to employ smaller size particles having diameter around few tenth nm, i.e., atoms. 

Here we consider some requirements on calixarenes as a nano-resist. The first to be considered is solubility, of course, to make films from solutions. Although Shinkai and his coworkers developed many water-soluble calixarenes, most of the calixarenes have poor solubilities either in water or in organic solvents and many of them are highly crystalline (1). An attempt to enhance solubility to organic solvents by substituting the upper rim with higher alkyl groups was in vain. 

Table 1 lists the values of the components obtained from the fit. The solution resistance was constant. The charge transfer resistance was a measure of the permeability of the membrane since it described the Faradaic current caused by the redox-active marker. A deerease of the charge transfer resistance by a factor of 2.1 was observed in the presence of 7.5 mM chloroform, confirming the inereased permeability of the membrane due to the calixarene-analyte interaction. However, the change of the charge transfer resistance was not as large as would have been expected from the results of the cyclic voltammetry (see above and Fig. 3c). This may be attributed to the increased irreversibility of the hydroquinone redox reaction under the influence of chloroform discussed above. 

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