High-contrast developers

Another method of increasing resist performance through the alteration of processing conditions involves the use of high-contrast developers (36). These developers are typically more-dilute aqueous-base solutions that improve selectivity between exposed and unexposed areas of the resist. Unfortunately, the high-contrast developers also tend to increase the time required for development. 

At temperatures below 50F/10C hydroquinone developers are inactive. It is extremely susceptible to the action of bromide. When compounded with alkali carbonates it gives a slow-working but high contrast developer. With caustic alkali (e.g., sodium hydroxide) its action is very rapid, with the highest possible contrast. For this reason, it is the most widely used developer for technical applications, especially in process work where the highest attainable contrast is essential. In the presence of caustic alkali (i.e., high pH) it is not temperature-sensitive and can be used for low-temperature developing. Hydroquinone developers keep well and are slowly exhausted. 

This is a high-contrast developer with good keeping properties and high capacity. It is especially recommended for continuous-tone scientific and technical work that requires higher-than-normal contrast. This developer has special applications for reversal processing (Reversal Processing, below). 

ASA films, such as Kodak Tri-X Pan for black and white prints or Ektachrome for color slides, are suitable for many applications. These films can be exposed at 800 or 1600 ASA and push-processed in a high-contrast developer. A combination that has found favor in many laboratories is that of Kodak Tri-X with the Diafine two-bath developer (1), which provides an ASA of 1600. In general, black and white film has a finer grain and is suitable in most situations, the exceptions being doublelabeling procedures in which both fluorophores are to be photographed simultaneously or the preparation of color slides for presentations. 

Phenidone, l-phenyl-3-pyrazolidone (738) (B-76M140404), has been used as a nonstaining, high contrast photographic developer. New optical brighteners containing 2-pyrazolines (739) and pyrazoles (740) have been synthesized recently and their properties and applications reviewed (75AG(E)66s). 

It can be seen in Table 1 that the lithographic sensitivity of the copolymers blended with 10% sulfonium salt increases as the percentage of latent electrophile (vinylbenzyl acetate) units is increased,. For a 50/50 copolymer the lithographic sensitivity is approximately 0.5 mJ/cm2 with a very high contrast of over 4. It should be noted however that aqueous development is no longer possible for the 50/50 copolymer for which some isopropanol must be added to the aqueous base developer. 

This system has been reported to show good photosensitivity (—100 mJ/cm2 in the 230-300 nm range) with extremely high contrast. Values of 7 > 5 were observed, compared to 2 for conventional systems. Chandross et al. believe the high contrast stems from the unusual dissolution characteristics of the resist in that the basic "developer" appears only to "prime" the irradiated regions for subsequent development in the aqueous rinse. The resist formulation is essentially aliphatic in nature and would be expected to be less stable to dry etching environments than the aromatic-based novolac resin materials. 

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