Design concept for a high-performance positive photoresist

Abstract

An attempt was made to design a high-performance positive photoresist from the standpoint of the image formation process. There are a lot of trade-off relationships among performance of a positive photoresist. A study was made to raise performance without the decrease in other performances. It was necessary to design a new type of novolak resin that has a molecular structure different from the existing materials. Many kinds of cresol–formaldehyde type novolak resins were synthesized and evaluated lithographically. Such items as molecular weight, molecular weight distribution, isomeric structure of cresol, the position of methylene bond, and the content of quinonediazide in a positive photoresist were investigated. Three methods were found useful: (i) to apply high-ortho novolak resins that have a high content of ortho–ortho’ methylene bonds (ii) to optimize molecular weight distribution of novolak resins, and (iii) to optimize the content of quinonediazides in a photoresist. These results are explained in terms of a simple model for positive photoresist development, which we call ‘‘stone wall model’’. This model is applicable to design a high-performance positive photoresist.