Negative resists polymers

Photoreactive chromophores based on pyridinium and other heteroaromatic bases have been incorporated into negative resist polymers as side chains. Styrl-pyridinium and styrylquinolinium are examples of such heteroatomic bases with photoreactive side chains. 

Shiriashi, Y. Taniguchi, S. Horigome, and S. Nonogaki, lodinated polystyrene An ion millable negative resist, Polym. Eng. Sci. 20, 1054 (1980). 

Negative resists generally exhibit high sensitivity but low contrast. For instp.ncin our laboratory, polymers containing thiirane groups / g v ere found to be extremely sensitive (o = 6 x 10 T C/cm2) to electron beam irradiation at a 20 kV... 

Polymer molecular properties such as molecular weight and polydispersivity have a significant effect on the lithographic behavior of the single component negative resists described above. For example, it has been shown for a series of... 

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. 

The value in units of incident dose per unit area for either a positive or negative resist system is of little value unless accompanied by a detailed description of the conditions under which it was measured. This description should include, at the minimum, the initial film thickness, the characteristics of the substrate, the temperature and time of the post- and pre-bake, the characteristics of the exposing radiation, and the developer composition, time and temperature. The structure, copolymer ratio, sequence distribution, molecular weight, and dispersity of polymers included in the formulation should also be provided. 

It should be noted that useful, high resolution (0.75 fim) patterns can be produced in certain negative resists if the development step is followed by a sequene of rinses in solvents that have less and less affinity for the polymer structure. The rinse steps effectively reverse the swelling that occurs during development. 

It was indicated earlier that swelling limits resolution in solvent-developed negative resists. It was also intimated that swelling effects could be minimized if there were a sufficient polarity change between the exposed and non-exposed areas of the type mentioned in the previous discussion of the PBOCST system. A similar principle was utilized by Hofer et al., (145-146) at IBM, based on ion pair formation. The resist consists of a polystyrene polymer to which tetrathiofulvalene (TTF) units have been attached. When spun down with an acceptor such as CBr4, a complex is formed which, on irradiation, undergoes an electron transfer reaction to form an ion pair ... 

Fig. 9. Negative resistance phenomena, a) 100 A thick polyethylene single crystal using Pt Catswhisker and Copper substrate [after van Roggen (725)], b) 70S A thick silicone polymer film at 300° K, c) 705 A thick silicone polymer film at 4° K [after Mann (726)]...

A distinction can be made between a positive and a negative resist, according to the action of light. In a positive working resist the monomer is deposited on the copper surface in the form of a viscous liquid. It is then irradiated through a mask (this is simply a drawing of the required pattern on a transparent sheet) and polymerization takes place only at the exposed places. The unirradiated liquid monomer is then washed away in a suitable solvent, and the exposed copper can be dissolved in an etching bath. Finally the protective polymer layer is removed by chemical or mechanical means, and the printed circuit is ready. 

Two types of resists are used negative and positive resists. Negative resists have a chemically inert polymer component, such as mbber, and a photoreactive agent that reacts with light to form cross-links in the mbber. When placed in an oiganic developer solvent, the unexposed, unpolymerized resist dissolves, leaving a polymeric pattern in the exposed regions. Because the polymer swells in the solvent, the resolution is limited to two to three times... 

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