Negative resists developer selection

Radiation-sensitive polymers are used to define pattern images for the fabrication of microelectronic devices and circuits. These polymers, called resists, respond to radiation by either chain scission (positive resists) or by crosslinking (negative resists). In positive resists, the exposed areas dissolve selectively by chemical developers in negative resists, the exposed areas are insoluble and remain after development. 

Figure 1. Diagram showing how irradiation through a mask allows selected areas of the photoresist to be exposed. In positive resists, the exposed areas become more soluble in the developer and, therefore, can be selectively removed. In negative resists, the exposed areas become less soluble in the developer, and thus, unexposed material is selectively dissolved.

Photoresists are divided into two classes. Negative resists are materials whose solubilities are decreased by exposure to actinic radiation. The image is then developed by selectively dissolving the more-soluble, unexposed portion of the film. This basic process was invented 150 years ago and it is used in the manufacture of printing plates (63). Positive resists, on the other hand, are materials whose solubilities are increased by exposure, so that in development the exposed portion of the film is removed. The history of photoresists is described in Kosar s book (63), which is also an excellent review of the history of many photosensitive materials. 

Positive Photoresists. Positive resists are entirely different from negative resists. For the purposes of this discussion we restrict ourselves to visible-light-sensitive materials. Typically, these materials are mixtures of low-molecular-weight phenol-formaldehyde polymers and derivatives of naphtho-1,2-quinone diazide, the photosensitive component. The former is soluble in aqueous alkali, but the presence of the latter, a hydrophobic species, inhibits attack of this developer on the film. On irradiation the "sensitizer" is converted to a ketene, which, after reaction with water, forms a base-soluble carboxylic acid. Thus the irradiated part of the film is rendered soluble in the developer and it can be removed selectively. The important feature of this system is that the unirradiated areas are not swollen by the developer and the resolution of this material is quite high. It is possible to prepare gratings having several... 

Figure 3.35. Negative plasma-development process via selective silylation of a positive-negative resist.

Resists function by radiation-induced alteration of the solubility of the materials. There are two basic classes of resist materials, namely, negative and positive resists (see Fig. 4.5). Negative resists become less soluble on exposure to radiation i.e., the unexposed areas can be selectively removed by treatment with an appropriate developer solvent. Positive resists selectively undergo an increase in solubility on exposure, enabling the exposed regions to be selectively removed in the developer. Both types of resists are formulated from polymers designed to have physical and chemical properties consistent with semiconductor... 

As stated earlier, when the exposed part of the negative resist film is developed in an appropriate solvent, areas that have not been exposed retain their original linear (or branched) solvency and are removed by the developer solution. Exposed areas, on the other hand, having been cross-linked, are able to resist the developer action, and are therefore not removed. In this way, a negative image is formed by the selective solvency of the exposed and unexposed areas in the developer. ... 

Positive resists differ from their negative counterparts principally in their response to actinic radiation, despite the fact that the essential composition of the two resist types are similar in many ways each contains sensitizers or appropriate radiation-sensitive compounds, resins, solvents, and additives. Unlike some negative resists, positive resists do not swell in developer. Moreover, the use of aqueous stripping and developing solutions greatly simplifies the equipment selection for positive resists in process equipment tooling hy allowing low-cost readily available plastics to be used as containers. Problems from the use of flammable solvents are minimized with positive resists. ... 

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